docs: fix three dead links in the install and developer docs

[rawsun007]

Three links in the published docs return 404.

docs/source/DEVELOPER-README.rst

• PHPUnit: phpunit.readthedocs.io/en/latest/writing-tests-for-phpunit.html → docs.phpunit.de/en/13.3/writing-tests-for-phpunit.html. PHPUnit moved off Read the Docs.
• phpDocumentor: docs.phpdoc.org/latest/guide/guides/running-phpdocumentor.html → docs.phpdoc.org/guide/getting-started/generating-documentation.html. The page was renamed; the new one is the same "how to run it" content, and it is the entry their current nav links.

docs/source/INSTALLATION.rst

• XAMPP: apachefriends.org/en/index.html → apachefriends.org/. The /en/index.html path is gone; the root serves the downloads.

Each replacement returns 200; each old URL returns 404.

What I deliberately left alone

• WampServer, on the line right after XAMPP. wampserver.com/en/ 404s, but so does wampserver.com/, so their site is down or blocking rather than restructured, and I would rather not repoint a link based on an outage.
• CHANGELOG.md references bookedscheduler.com/phpscheduleit, also dead. That is the historical changelog, so rewriting old entries would falsify the record.
• The XAMPP link text reads XAMMP (transposed). I left it, since this PR is scoped to URLs — say the word and I will fix the spelling too.

Per the AI-assistance rule in CONTRIBUTING.md, both commits carry Assisted-by: Claude Code:claude-opus-5. Every URL claim above was verified with a request rather than inferred. Based against develop.

[rawsun007]

Sorry for the noise — the first push failed three checks and all three were my fault. Now green (17/17).

• sphinx and doc8 both flagged DEVELOPER-README.rst:91: Title underline too short. Those link lines are section titles, and my longer phpDocumentor URL outgrew the ~~~ underline beneath it. Underline extended to match the title exactly.
• lint-commits flagged body-max-line-length on both commits; the bodies are now wrapped at 72.

While fixing the commit messages I reset the branch to base, which made GitHub auto-close this PR since head briefly equalled base. That is why it shows closed and reopened — the diff is unchanged in intent: three URLs plus the underline.