dbt-docs changes for v0.18.0 (final)

[jtcohen6]

resolves #2705 (again), follow-up to #2715

Description

• Final docs change for v0.18.0: support for displaying project-level overview pages. I've tested this locally, it's additive only, I just need to document it
• Add changelog entry for docs#121. This change was included in the RC, but I missed it in the changelog when I retroactively wrote up the changes in Docs site updates for 0.18.0 #2715. The remedy here is that each PR to the dbt-docs repo will be required to add its own changelog entry in that repo. I will cut releases in that repo, then open PRs (like this one) to port over the changes + changelog.

Checklist

• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have updated the CHANGELOG.md and added information about my change to the "dbt next" section.

[beckjake]

lgtm, though obviously I have not examined the contents of index.html