New docs for context creation and initialization

Draft

Fixes #594
Part of #876
Fixes #1390
Fixes #1923
Fixes #2241
Part of #2523
Part of #2549
Part of #1832

Let's discuss where this page should go and what goes underneath it. We had previously decided to keep this in Miscellaneous, but I now believe this would be better as a top-level item with the following subsections to go into more detail:

- DbContext lifetime, configuration and initialization
  - Overview
  - Connection strings
  - Using dependency injection
  - Connection resiliency
  - Context pooling