Clarify Read the Docs versioning and retention of older versions

[daksh1403]

What this PR does

Clarifies that mypy documentation on Read the Docs is intended to be
versioned using released git tags, and that older documentation versions
are kept available.

Why

Read the Docs supports documentation versioning for community projects.
Using git tags avoids relying on long-lived maintenance branches while
still allowing users to access documentation for older mypy releases.

Scope

Documentation-only change. No impact on mypy runtime or type checking.

Fixes #19922

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[wyattscarpenter]

I don't see how this would actually fix #19922 unless some other change is made to actually accomplish this goal.

[daksh1403]

I don't see how this would actually fix #19922 unless some other change is made to actually accomplish this goal.

Good point, this PR is documentation-only. It doesn’t change RTD behavior, but clarifies that versioning is based on git tags and that older docs are expected to remain available. The intent is to address the confusion raised in #19922 rather than implement a tooling change. Happy to adjust scope or issue linkage if needed

[sterliakov]

I'm afraid that the text proposed in this PR is also wrong? Right now only two version aliases - "stable" and "latest" - are available on RTD. Being able to fetch a git tag and build RTD pages locally does not constitute easy versioned docs access, and your text seems to suggest that one can go to RTD and find the desired version by git tag there.

[wyattscarpenter]

@sterliakov he's basically proposing someone fix #19922, after which his proposed text will be true (although possibly not worth mentioning anyway...)

[daksh1403]

Thanks @sterliakov for catching that, and @wyattscarpenter for clarifying my intent
You are exactly right this documentation update anticipates that we will enable the git tag versions on Read the Docs to resolve #19922. Without that configuration change, this text is indeed incorrect.
Since I don't have admin access to the Read the Docs dashboard, would a maintainer be open to enabling the builds for released git tags? If we can flip that switch, this PR effectively documents the new capability and resolves the confusion raised in the issue.