chore: retire configdocsgen

[alexluong]

Takes the "retire the generator" option from #820.

cmd/configdocsgen reflected over internal/config and injected generated env-var and YAML blocks into docs/pages/references/configuration.mdx, between MDX placeholders. That page went away with the move to Markdoc, so the tool wrote to a path that no longer exists:

$ make docs/generate/config
Warning: Output file docs/pages/references/configuration.mdx does not exist. A new file will be created with placeholders.
Error generating docs: failed to write updated content to docs/pages/references/configuration.mdx: open docs/pages/references/configuration.mdx: no such file or directory
exit status 1

Its replacement, docs/content/self-hosting/configuration.mdoc, is curated — grouped into sections with prose around them, a different shape from the generator's single table — and carries no autogen placeholders. There is nothing left for the generator to fill in.

Changes

• Delete cmd/configdocsgen.
• Drop the docs/generate/config Make target.
• contributing/config.md step 6 now says to edit configuration.mdoc by hand. It previously pointed at go generate ./internal/config/..., and the //go:generate directive it names no longer exists either.

The historical plan under contributing/.plans/ still describes the generator; left as-is since it is a dated record.

Closes #820

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