Consolidate best practice documentation

[iaindillingham]

We have several issues and discussions that relate to documenting best practice:

• #688
• #627
• #594
• #252
• #46
• #45
• #801
• #795

Currently, best practice is located under OpenSAFELY best practice in the menu. However, this is a pick-and-mix: from the manifesto to a study design, via Git. There is also a section on Programming tricks, although this relates to study definitions; and we have the discussion/Q&A forum.

It would be good to address these issues and to consolidate best practice documentation.