Cleanup docs

[braingram]

Description

Closes #1893

Updates docs from review.

Some changes of note are:

• removed outdated developer overview. Much of this file contains descriptions of the long-removed legacy extension API and other pre-asdf-3.0 code.
• fix several places where the old default of memmap=True was described
• fix several issues with the FractionWithInverse converter example
• simplify our release and dependency support document referencing the now-established https://scientific-python.org/specs/spec-0000/

Tasks

• run pre-commit on your machine
• run pytest on your machine
• Does this PR add new features and / or change user-facing code / API? (if not, label with no-changelog-entry-needed)
  • write news fragment(s) in changes/: echo "changed something" > changes/<PR#>.<changetype>.rst (see below for change types)
  • update relevant docstrings and / or docs/ page
  • for any new features, add unit tests

news fragment change types...

• changes/<PR#>.feature.rst: new feature
• changes/<PR#>.bugfix.rst: bug fix
• changes/<PR#>.doc.rst: documentation change
• changes/<PR#>.removal.rst: deprecation or removal of public API
• changes/<PR#>.general.rst: infrastructure or miscellaneous change

[zacharyburnett]

some suggestions

[nden]

This looks good to me.
Question about the developer docs - are they still valid/necessary or obsolete? Should they be moved somewhere else?

[braingram]

This looks good to me. Question about the developer docs - are they still valid/necessary or obsolete? Should they be moved somewhere else?

Thanks! That document is largely out-dated. Reading through it recently it contains:

• descriptions of the extension API which is documented as part of the public API
• descriptions of the legacy extension API which no longer exists
• descriptions of internal workings of parts of asdf which are out of sync with the code, maintaining separate private documentation doesn't seem worth the effort when the code and private docstrings can be consulted

I'm in favor of removing this document and not updating or replacing it. If there are gaps in the private documentation adding docstring and comments to the non-obvious code seems like an approach that would be more likely to be maintained with future code updates.

[braingram]

@nden I went ahead and merged this. If updating/restoring or an alternative for the removed developer docs is wanted I can open a follow-up PR.