Automatically include integration READMEs on docs site

[delucis]

• Waiting for Integration Docs Next Steps astro#3677 to merge.
• (And non-blocking, Updates integration package READMES to remove "experimental" warning from astro add command astro#3727)

Random Reminders

• Include link to relevant starter package/astro.new?? (For playing around, seeing what it should look like if it works, minimal reproductions if trouble)

Notes from Chris’s sync w/ Tony, 7/7/22

• Add integration READMEs to Docs — only official @astrojs ones for now
• Show links using a deploy-style icon grid in the Integrations Guide
• Link to new pages from the cards on https://astro.build/integrations/ (maybe straight update the URLs in each integration’s package.json so we also link to docs from npm?)
• Follow same pattern as for Configuration Reference:
  • fetch READMEs in a nightly CI task
  • generate .md pages from those
  • open a PR with any changes
  • That will trigger the i18n process like any other page and gives us a chance to label the PR as typo/minor etc. if we need to