Document EF Core architecture and internal technical details

To make it easier for contributors and provider authors to understand the code.

**Please comment on this issue with internal areas you would like to see documented.**

Working list of possible topics to document. This list will evolve in the issue as we make progress. Not everything in the list will necessarily make it to documentation--it will depend on value, resources, etc. as usual.

- [ ] Internal service provider
- [ ] Singleton options
- [ ] Dependency objects
- [ ] DbContext initialization
- [ ] Context pooling
- [ ] Change tracking
- [ ] SaveChanges/update pipeline
- [ ] Everything query!
- [ ] Proxies
- [ ] Lazy-loading
- [ ] Constructor injection
- [x] Everything model building! (https://github.com/aspnet/EntityFramework.Docs/issues/779)
- [x] Model building conventions (https://github.com/aspnet/EntityFramework.Docs/issues/1937)
- [ ] Shadow properties
- [ ] Logging
- [ ] Interceptors
- [ ] Diagnostics
- [ ] External model building
- [ ] Metadata interfaces (mutable, etc.)
- [ ] Weak entity types
- [ ] Owned entity types
- [ ] Relational mapping (https://github.com/dotnet/efcore/issues/12846)
- [ ] Pubternal types
- [ ] Parameter generation and facets