Doc: add anchormap directive for anchor redirects

[StanFromIreland]

See python/docs-community#134 (and python/docs-community#58) for prior discussion and motivation. The .. anchormap:: lets us declare old-anchor: :ref:`new-target` entries, which the anchor_redirects.py extension resolves at build time and embeds as JSON in the page. A small Javascript script then redirects any achor to its new location.

To see it in action, go to: https://cpython-previews--151113.org.readthedocs.build/en/151113/using/configure.html#cmdoption-with-system-libmpdec

For that example, the extension added to using/configure.html:

<script id="python-docs-anchor-redirects" type="application/json">{"cmdoption-with-system-libmpdec": "../whatsnew/3.16.html#rem-bundled-libmpdec"}</script>
<script type="module" src="../_static/anchor_redirects.js"></script>

In this PR I only removed one entry from removed-ids.txt to serve as an example, the rest can be done in follow-ups.

[read-the-docs-community]

Documentation build overview

📚 cpython-previews | 🛠️ Build #33047621 | 📁 Comparing e6d0bbe against main (29a920e)

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1 file changed

± whatsnew/3.16.html

[encukou]

Sorry to be negative, but I find the automatic redirect rather confusing. Especially with your example (the mpdec option) on a big screen -- there's not much indication that the thing you were linked to is now gone.

I'm not sure this iteration is a net positive, let alone worth the maintenance cost :(

[StanFromIreland]

Sorry to be negative, but I find the automatic redirect rather confusing. Especially with your example (the mpdec option) on a big screen -- there's not much indication that the thing you were linked to is now gone.

That's generally how redirects are implemented, although I agree it's not ideal. The rediraffe extension and the various nginx redirects we use work the same way (e.g. https://devguide.python.org/contrib/project/github.html redirects to the main index with no note). What if we added a small pop-up to notify the user of the redirect, it is done client-side so it wouldn't been terribly difficult?

I'm not sure this iteration is a net positive

I see, in any case I still think we need to do something. As it stands, the CI check is a really just a doctor who diagnoses you and walks out. ;-)