GRO-838 Adds support for _includes in docs src

[bpander]

GRO-838

⚠️ The _docs changes are for demo purposes only. I'll remove 3f6e2a3 before merging. ⚠️

Description

Allows includes from metabase docs src files so tech writers can reuse code that lives entirely in the mb repo.

Adding this to a [mb repo]/docs file will include [mb repo]/docs/_includes/test-md.md

{% include {{ _includes }}/test-md.md %}

Relevant slack thread

How to verify

• Files that live in [mb repo]/docs/_includes can be included from other files in [mb repo]/docs
• Both md and html includes should be supported
• Astro should not generate pages for files in _includes
• sitemap.xml should be unchanged (should not include entries for _includes)
• script/docs should not add frontmatter to _includes
  • test locally with ./script/docs -r "../metabase" --latest --update (-r is wherever your local metabase repo is)
    • FYI this script will go a lot faster once GRO-829 Remove embedding docs generation #1293 is merged
• Build times should not be meaningfully impacted
• Builds should fail if the include can't be resolved

Demo

https://gro-838-docs-includes.dkkh35gceu2po.amplifyapp.com/docs/latest/embedding/authentication

https://gro-838-docs-includes.dkkh35gceu2po.amplifyapp.com/docs/v0.63/embedding/authentication

[linear]

GRO-838

[iethree]

Very snazzy. Makes a lot of sense short term.

Longer term, instead of committing to liquid syntax, might we want to move to .mdx + react style components? If we do, does it make sense to support this in the docs only to refactor later?

[bpander]

does it make sense to support this in the docs only to refactor later?

I had the same thought last night. I also want to see what this looks like with mdx instead of having to support a new variable (_includes) in perpetuity. I'll move this back to draft for now.