docs: syntax highlighting for (some) code examples

[clason]

Feature already in Vim?

No!

Feature description

Now that we bundle a (mostly) working treesitter parser for help files, we can have nice things: properly syntax-highlighted code examples. This involves the following steps:

• extend the vimdoc parser to support language annotation for code markers (e.g., >lua) and add the corresponding injections to the bundled queries (feat: annotated code blocks tree-sitter-vimdoc#65)
• add corresponding conceal rules to syntax/help.vim to fall back gracefully if treesitter is disabled (feat(help): highlighted codeblocks #21154)
• adapt the HTML documentation generator script to turn annotated blocks into highlighted code blocks (using https://highlightjs.org? feat: add highlight.js neovim.github.io#303)
• successively rewrite the (Neovim-specific) help pages to add the right language annotations (start with all files titled "NVIM REFERENCE MANUAL")
• change gen_vimdoc.py to handle annotated <pre> codeblocks
• add annotation to <pre> blocks in Lua and C docstrings
• profit!