docs: fix dead virtualenv discovery link in explanation.rst

[CAOShurong]

• ran the linter to address style issues (tox -e fix)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation

Problem

While reading docs/explanation.rst I noticed the virtualenv discovery logic link 404s.

The virtualenv documentation was restructured: the old
https://virtualenv.pypa.io/en/latest/user_guide.html#python-discovery page
no longer exists.

Verified with live requests:

GET https://virtualenv.pypa.io/en/latest/user_guide.html#python-discovery  -> 404
GET https://virtualenv.pypa.io/en/stable/how-to/usage.html#select-a-python-version -> 200

Solution

Point the link at the current virtualenv docs location for interpreter
discovery (how-to/usage.html#select-a-python-version) and add the required
documentation news fragment.

Verification

• The replacement URL returns HTTP 200 and contains the
  select-a-python-version anchor.
• The other two virtualenv.pypa.io/en/latest/... links in the repo
  (reference/compatibility.html, changelog.html) still resolve, so only the
  removed user_guide.html page is addressed here.
• tox run -e docs passes.
• tox run -e fix passes.
• The changelog fragment passes the repository's targeted pre-commit hooks.
• The current tox env type CI failures are unrelated dependency drift:
  ty 0.0.75 newly warns on the pre-existing docs/conf.py:176, while the
  same base passed with ty 0.0.74; this PR does not touch docs/conf.py.

This is a documentation-only change; no runtime code is affected.

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_{This contribution was prepared with the assistance of an AI coding agent, used as a tool under my direction; I reviewed and verified the change myself.}