docs: integrate extraneous text on positions somewhere in the docs

[claymcleod]

As I was writing and editing the section on positions, I decided to remove quite a bit of content from the initial draft of the crate documentation. This content is useful in scope, but it may not be 100% correct as included here (the style I used when writing the crate was writing my own understanding of certain things and then doing quite a bit of research afterwards to ensure my statements were true). In this case of this content, some thoughts are even outright incomplete.

//! #### Extra
//!
//! * The notation 0-based and 1-based is not good because it takes away focus
//!   from the part of the intuition that actually matters (interbase vs
//!   in-base). Further, the assignment of interbase indices starting a 0 and
//!   in-base positions starting at zero in a informed, yet arbitrary, decision
//!   that is simply held together by convention.
//! * When you break the model down into nucleotide slots and space slots, it's
//!   clear that distinctions between "fully-closed" and "half-open" intervals
//!   are artifacts of trying to make space + nuceltodie slots fit into one
//!   concept, and they are only helpful in the context of resolving the
//!   interval to a nucleotide sequence.
//!     * The situation for in-base coordinates is quite clear: they include
//!       both the start and end nucleotide, and there is no compelling reason
//!       to do otherwise.
//!     * In a strict sense, whether interbase intervals are closed or open is
//!       undefined: this is largely due to that fact that we mostly care about
//!       resolving to nucleotide sequences and the spaces are filtered out, so
//!       we don't care if the last space is included or not (it has no bearing
//!       on what nucleotides are included in the sequence).
//! * Because interbase coordinates do not actually represent nucleotides, they
//!   must be resolved in a left-learning or right-learning manner (which is an
//!   assumption that should be explicitly denoted). In the UCSC model, they are
//!   silently assumpted to be right leaning, but the interval could be resolved
//!   by being left leaning and inclusive. They never mention this assumption,
//!   so it's a bad thing.
//!
//! #### Appendix
//!
//! Below are some scattered thoughts—many of which were written during the
//! original authorship of this documentation—that are related to genomic
//! coordinate systems. They are all tangentially related to the content above
//! but, during the editorial phase, were deemed to not fit the flow and purpose
//! of the documentation above. That said, they are still useful for individuals
//! who really want to dig into coordinate systems or design your own, so they
//! will remain included here. **You can skip this section if you're just
//! interested in learning about the crate.**
//!
//! ##### Intuitions That Arise From Transforming In-Base To Interbase
//!
//! Let's review the diagram of the conceptual model of coordinate systems.
//!
//! ```text
//! ========================== seq0 =========================
//! •   G   •   A   •   T   •   A   •   T   •   G   •   A   •
//! ║   ║   ║   ║   ║   ║   ║   ║   ║   ║   ║   ║   ║   ║   ║
//! ║[--1--]║[--2--]║[--3--]║[--4--]║[--5--]║[--6--]║[--7--]║ In-base Positions
//! 0       1       2       3       4       5       6       7 Interbase Positions
//! ```
//!
//! ```text
//! ========================== seq0 ===========================
//! •   G   •   A   •   T   •   A   •   T   •   G   •   A   •
//! ║       ║       ║       ║       ║       ║       ║       ║   In-base Positions
//! 0------>1------>2------>3------>4------>5------>6------>7-- Interbase Positions
//! ```
//!
//! Looking at this diagram and considering the ranges that represent the entire
//! sequence of nucleotides within both coordinate systems, it's clear that the
//! transformation from an in-base coordinate system can be broken down into two
//! main steps:
//!
//! * The range is shifted forward by half a step (one slot), and
//! * A whole step is prepended to the coordinate system.
//!
//! Another way of conceptualizing this is that the start and end of the in-base
//! range are expanded outward by one half step (one slot), though this
//! intuition doesn't naturally explain why the in-base coordinate system starts
//! at `1` and the interbase starts at `0`.
//!
//! This leads to a couple of key points:
//!
//! * Under both intuitions, the result is that the size of the range increases
//!   by one whole step. This is the key reason why the length of the range is
//!   "easier" to calculate in the interbase system (the length of the range is
//!   simply `end - start`, and you don't have to remember to add `1` to the
//!   result).
//! * If one chooses to combine the space slots and the nucleotide slots
//!   together as a single numbered position (as is often done in practice),
//!   each representation contains both a space slot and a nucleotide slot, but
//!   the order is switched:
//!     * The space slot is _before_ the nucleotide slot within the interbase
//!       system, but
//!     * The space slot is _after_ the nucleotide slot in the in-base system.
//! * The above point is, at least in part, the reason why combining the two
//!   slots together within a singular, numerical position often proves to be
//!   difficult to reason about: the rules change between the two coordinate
//!   systems, and one has to keep that accounting in mind.