Add best bets to the developer docs

[jonathanglassman]

What

Update (and merge?) the best bets guidance on the Content Support wiki:

https://gov-uk.atlassian.net/wiki/spaces/CC/pages/122093585/Amending+search+results+best+bets

https://gov-uk.atlassian.net/wiki/spaces/CC/pages/42303723/Change+internal+search+results+best+bets

Why

Search Admin has new features including expiry dates for best bets, which aren't reflected in the existing guidance.

Some of the guidance refers to old teams that no longer exist.

It's done when (acceptance criteria)

• We've reviewed the draft updates with Jon G.
• We've published the updated guidance.

Trello card: https://trello.com/c/ERdVWzdf/522-update-wiki-guidance-on-search-best-bets