Improve documentation on how to write an EF Core provider

[divega]

Currently we have something very brief at https://docs.microsoft.com/ef/core/providers/writing-a-provider pointing to @ajcvickers's blog.

It would be good to update this to cover:

• Support for EF Core 2.1 features
• The mechanics of setting up spec tests (e.g. https://github.com/dotnet/efcore/issues/27107#issuecomment-1004923695)
• The mechanics of adding provider specific extension methods

Whether this should be in a blog post or if it needs to be part of the documentation is something we can discuss, but I would be inclined to have it in the actual documentation.