Expand darktable User Manual coverage of "custom sort"

[wbclay]

Please include a link to the Pull Request that you are documenting

#723

Tell us a little bit about this pull request

These mods are intended to fully resolve dtdocs issue #672. They contain a
comprehensive description of darktable's "custom sort" sequencing feature.
They reflect trial-and-error testing on DT v.5.0.0 and study of its database.
They should be reviewed by those knowledgeable of the relevant DT internals.

Custom sort may not not be DT's hottest feature, but it's an elegant mechanism
that deserves to be presented clearly. Many users are likely to find custom
sort very useful if they understand how to use it and are aware of a few
pitfalls (probably fixable in the future).

This pull request comprises mostly minor and easy-to-merge edits of 12 current
.md files as well as 2 new .md files.

Comprehensive change list by section number:

1.1.3, ./overview/user-interface/filmstrip.md
existing ¶ 2, append sentence w/link re. "custom sort" drag-and-drop

1.1.4, ./overview/user-interface/top-panel.md
existing filter/sort item, split items, edit each, link to fundamental concepts

1.3.1, ./overview/sidecar-files/sidecar.md
existing 1st sentence, insert footnote [^1] reference
add new footnote [^1], "custom sort" not saved in sidecar files (and why)

1.4.4, ./overview/workflow/export.md
existing list, insert new item 3, use of $(SEQUENCE) w/"image sequence" link

2.1, ./lighttable/overview.md
existing intro to "# keyboard shortcuts", consistency edit for additional item
add new item "drag and drop" w/two subordinate items

2.3, ./lighttable/undo-redo.md
existing 1st sentence, insert footnote [^2] reference
add new footnote [^2], no "Undo" for drag-and-drop "custom sort" order changes

2.4.1, ./lighttable/lighttable-modes/filemanager.md
existing item "select", clarity edit
add new items "hover" and "drag"

2.4.2, ./lighttable/lighttable-modes/zoomable-lighttable.md
existing item "select", clarity edit
add new item "hover"

2.5.5, ./lighttable/digital-asset-management/metadata-tagging.md
existing sentence after "# tagging," insert link, topic "image sequence"

2.5.6 (new), ./lighttable/digital-asset-management/sequence.md
add file, weight 70: new topic "image sequence"

2.5.7 (new), ./lighttable/digital-asset-management/custom-sort.md
add file, weight 80: new topic "custom sort"

8.3.5.3, ./module-reference/utility-modules/shared/export.md
existing item "filename template", insert sentence, $(SEQUENCE) example use

8.3.5.4, ./module-reference/utility-modules/shared/filmstrip.md
existing ¶ 3: replace last sentence, redundant with ¶ 4, 2nd sentence; add new last sentence re. "custom sort" w/link
existing ¶ 4, 2nd sentence: "different" image for k/b shortcuts; clarity edit

8.3.5.11, ./module-reference/utility-modules/shared/tagging.md
existing item "tag", append new sentence w/link re. "custom sort"