docs: add PostHog AI plugin to MCP docs

[MattBro]

Changes

Adds the PostHog AI plugin to the MCP documentation pages:

• Claude Code page: New "Claude Code plugin" section with install command, OAuth auth instructions, and available slash commands
• Codex page: New "Codex plugin" section with install command
• MCP index page: Link to the ai-plugin repo in "Next steps"

Requested in PostHog/ai-plugin#13 (comment)

Checklist

• I've read the docs and/or content style guides.
• Words are spelled using American English
• Use relative URLs for internal links
• I've checked the pages added or changed in the Vercel preview build
• If I moved a page, I added a redirect in vercel.json

[github-actions]

Deploy preview

Status	Details	Updated (UTC)
🟢 Ready	View preview	Apr 03, 2026 09:01PM

[github-actions]

Vale prose linter → found 0 errors, 4 warnings, 0 suggestions in your markdown

Full report → Copy the linter results into an LLM to batch-fix issues.

Linter being weird? Update the rules!

contents/docs/model-context-protocol/claude-code.mdx — 0 errors, 2 warnings, 0 suggestions

Line	Severity	Message	Rule
9:147	warning	Capitalize 'Feature Flags' for PostHog's product. Use 'feature flags' for the general industry concept.	PostHogBase.ProductNames
26:4	warning	'Claude Code plugin' heading should be in sentence case, and product names should be capitalized.	PostHogBase.SentenceCase

contents/docs/model-context-protocol/codex.mdx — 0 errors, 2 warnings, 0 suggestions

Line	Severity	Message	Rule
9:141	warning	Capitalize 'Feature Flags' for PostHog's product. Use 'feature flags' for the general industry concept.	PostHogBase.ProductNames
36:111	warning	'OAuth' is a possible misspelling.	PostHogBase.Spelling