Reinstate #backport-pr-title anchor and add explicit backport PR title guidance

[hugovk]

When Bedevere fails a backport PR because it's missing a [3.x] title prefix, it links to the devguide with advice.

The old link:

• https://devguide.python.org/committing/#backport-pr-title

Redirects to:

• https://devguide.python.org/core-team/committing/index.html#backport-pr-title

But since #1819, there's:

• no #backport-pr-title anchor on that page
• the backporting guide was moved to another page, with a stub also pointing there

For reference, the old devguide pointed to:

• https://web.archive.org/web/20251002203521/https://devguide.python.org/core-team/committing/#backport-pr-title

This PR:

• re-adds #backport-pr-title to the original page, at the stub
• adds some explicit backport PR title guidance

[read-the-docs-community]

Documentation build overview

📚 CPython devguide | 🛠️ Build #33511870 | 📁 Comparing f571373 against latest (4ea7395)

  🔍 Preview build  

3 files changed

± versions/index.html
± core-team/committing/index.html
± getting-started/git-boot-camp/index.html

[StanFromIreland]

LGTM w/ one little suggestion.