Remove legacy documentation from search

[LFISHER7]

There is legacy documentation relating to cohort extractor (e.g. https://docs.opensafely.org/study-def/) that has some overlap with other content. Both appear in search results, which can be confusing (see this slack thread).

We could maintain the legacy documentation but remove it from search results using this mkdocs plugin.

[StevenMaude]

So looking around, it is possible to remove the pages from search results in Material for MkDocs by editing the page itself:

https://squidfunk.github.io/mkdocs-material/plugins/search/#meta.search.exclude

The plugin that Louis linked maybe makes it a bit more obvious in the configuration that a subset of pages are excluded. But if we only have the eight pages of the "Legacy" section to exclude, so maybe it's OK to do that per page, rather than add another plugin.

(I guess there's an associated question of: do we want to remove these results from search engines. But I think probably not.)

[lucyb]

Because this refers to Cohort Extractor, I'm putting this into Team RAP's triage list for them to look at.