Add php-flat-file to Static Site Generators

[slogsdon]

Adds php-flat-file under Static Site Generators, alphabetically between Jigsaw and Sculpin.

It is a flat-file CMS and static-site generator with no database, no configuration, and Markdown content. MIT licensed, installable via Composer (slogsdon/flat-file on Packagist).

Adheres to the contribution guidelines:

• Markdown format [LIBRARY](LINK) - DESCRIPTION.
• Description ends with a period and contains no trailing whitespace
• Added alphabetically to the category
• Not a duplicate

[slogsdon]

Heads-up: the previous CI run failed on a pre-existing entry, not on the addition.

Link Check reported one error: https://phpdox.net/ | Network error: received fatal alert: InternalError. That's a server-side TLS handshake failure on phpdox.net (reproducible from a local curl), not an intermittent flake. That domain has currently broken TLS.

Pushed a separate commit swapping the phpDox entry to its canonical GitHub repo (https://github.com/theseer/phpdox), which 200s and matches how the surrounding documentation entries link out. The new commit is awaiting workflow approval since it's a first-time contribution. Happy to drop the phpDox commit if you'd rather land it via a separate PR.