Add option to include posts in search index

[mrpsharp]

This adds a search option to include the Jekyll posts in the search index without creating Just The Docs collections. (Reopening #812)

[SgtSilvio]

Hi @mrpsharp this should already be possible by setting the following config:

just_the_docs:
  collections:
    posts:
      nav_exclude: true

This will add the _posts collection to search (because search_exclude is not set) but will not include the posts in the navigation. With that, no special code changes should be required.

[mattxwang]

@mrpsharp does @SgtSilvio's suggestion address your use-case? Not sure how much we explored the problem space before opening the PR.

[mattxwang]

Hm, atm going to close this due to inactivity. Feel free to re-open / discuss further if there's still interest!