Add Markdown output format for PHP documentation

[philip]

Adds a new markdown output format to the PHP package, producing clean Markdown from PHP DocBook sources.

Output:

• One .md file per chunk, named using the same convention as XHTML and Web (function.array-map.md, class.exception.md, etc.)
• PHP 8-style function/method signatures in fenced php code blocks
• Class declarations with modifiers, extends, and implements
• GFM tables for changelogs and parameter tables
• Blockquote admonitions (> NOTE:, > WARNING:, etc.)
• Relative .md links throughout, matching XHTML link structure
• Enum cases in fenced code blocks

New files:

• Package/PHP/Markdown.php: the format renderer
• PI/MarkdownPHPDOCHandler.php: changelog PI handler (GFM table output)

Usage:
phd -f markdown -P PHP -d doc-base/.manual.xml -o out/

[Crell]

What is the use case for this? Normally markdown is a source format for producing HTML, and there is already an HTML output format.

[philip]

What is the use case for this? Normally markdown is a source format for producing HTML, and there is already an HTML output format.

For agents, mostly. I understand this would means a discussion, but one general idea is to allow agents (or humans) to view Markdown. In two forms:

• Simple extension change, example: https://www.php.net/manual/en/language.variables.basics.md
• Or standard URL but with Accept: text/markdown header.

[hakre]

What is the use case for this? Normally markdown is a source format for producing HTML, and there is already an HTML output format.

HTML is not actionable, MD is. @Crell

[Crell]

What does actionable mean? Do you mean LLM friendly? Those can go to hell, I have no interest in giving free labor to criminals.