Update PyPI project links

[rmartin16]

Changes

• The Documentation link is updated to the default landing page configured in RTD...which is currently stable
  • Previously, this was latest but that seems strange given these links only really show up on PyPI...and the code on PyPI usually isn't the "latest"
• Add a new "Changelog" link on PyPI for Toga's release history
  • I had previously mentioned linking to the GitHub release page to advertise new releases but it was said that PyPI is the official source of releases
  • That makes sense to me but I've been a little bothered that the release history isn't prominently made available to users on PyPI; this resolves that with a dedicated link
  • The name "Changelog" for the link probably has room for debate...
• Update RTD link in READMEs to latest instead of stable
  • If you're reading a README, you are probably more interested in the latest documentation than the last release

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[rmartin16]

Example of these changes on PyPI:

https://test.pypi.org/project/x-toga-core-20240417/0.4.3/

Using the name "Changelog" may make this a little too prominent perhaps since PyPI seems to alphabetically sort these links....so, something like "Release History" would move it down...

[EDIT]
These are the expected names for a "changelog" link on PyPI:

["changelog", "change log", "changes", "release notes", "news", "what's new", "history"]

[freakboy3742]

Makes sense. I don't think there's any single canonical answer on which version of the docs is best to link. If you're viewing the README on Github, you likely want the latest; but if you're on PyPI, you probably want the stable (and, more specifically, the stable for that release). However, I can't think of an easy way to automate that configuration, and I wouldn't have thought PyPI is a common launching point for README content - or if it is, they're going to use the "Documention" link on the side which will point at stable, so I'm not overly concerned about this.

As for changelog - I don't think it's worth getting bent out of shape over ordering.