Show a template's documentation and its text on its own page

[bonelifer]

The page of a Template: article now makes sense. It used to show <noinclude> as text and work out its own calls with no parameters, so an infobox template showed up scrambled.

• Documentation. What stands inside <noinclude> is shown as the page's text, rendered like an article, with its calls worked out so it can show examples.
• Template text. Below it, the text an article gets (the page without <noinclude>, and with <includeonly> tags removed) is shown in a code box as it is written, not worked out.
• Use count. A line says how many articles use the template ("Used in 1 article.", or "Not used in any article yet."). What links here still lists them.
• <includeonly> text is left off the page, and no <noinclude> or <includeonly> tag is shown. Ordinary articles are unchanged.
• README and the TODO plan are updated.

Tests

• Unit tests: 221 tests, 514 assertions, all pass. New case for reading the documentation parts.
• A new browser test passes 9 of 9 on phpBB 3.3.19 with SQLite, 4.0.0-a3-dev with SQLite and PostgreSQL 16, and on MySQL 8.0 as well; it covers documentation with a live example, the unworked template text, the use count and the list, an ordinary article, and an unused template.
• The earlier template browser test (its check of the template page updated) passes on all four boards.

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Investigated and written by Claude on behalf of William Jacoby (bonelifer).