Document a deprecation strategy

[MVrachev]

Description of issue or feature request:
With the release of python-tuf 1.0.0 and the soon integration of python-tuf inside Warehouse we should decide and document what will our depreciation strategy be.

For example: in my pr #1948 @jku has suggested that we should deprecate DelegatedRole.is_delegated_path() API call.
I have a couple of questions:

1. How do we document this in the function docstring?
2. Where else do we want to document the depreciation?
3. How many versions of python-tuf we will keep the deprecated method before its removal?
4. Maybe it will be good to have the process to create an issue for each API call that we want to deprecate and that way we can keep track of the APIs we want to deprecate?

Current behavior:
No depreciation strategy.

Expected behavior:
Define one and maybe document it in an ADR and start following it.

[MVrachev]

On point 4 - having a separate issue about the deprecation of an API will be like an announcement and will give the users the opportunity to complain, ask for more versions to support the API, and a place for a discussion about when should this API be removed.

[ivanayov]

Related to #1127