(docsdev) WEB-6269: Add Further Reading if present in manifest (Part 1)

[hestonhoffman]

What does this PR do? What is the motivation?

Adds a further reading partial if the integration has the "marketplace" category and further reading entries are present in the manifest/frontmatter.

This is intended as a follow-up to these PRs. It will only work once they're merged.

• Supporting marketplace integrations from APW #28811
• https://github.com/DataDog/websites-sources/pull/223

Also relies on:

• https://github.com/DataDog/websites-sources/pull/272: Removes any existing Further Reading sections from existing integrations.

Merge instructions

Do not merge

Testing

1. If you haven't already, clone the websites-sources repo and fetch/checkout the branch heston/heston/WEB-6269-futher-reading.
2. In your go.mod file, uncomment the replace github.com/DataDog/websites-sources line and point it at your local websites-sources repo.
3. Run make start.
4. Go to http://localhost:1313/integrations/bordant-technologies-camunda/. There should be a further reading link at the bottom of the page