Linting for title field

Moving to Hugo includes moving page titles from h1 markdown elements to title front matter attributes.

We will need a way to test for this and enforce it as part of docs CI pipelines. This may also require an adjustment to our markdownlint rules.

Whatever work we do to support this should merge in the same MRs that modify page titles. This may be part of the same MR for updating index file names, but TBD.