Fixed documentation and adjusted API for 2.0 release.

[xelandernt]

Goes through the points in #142:

• Tested 2.0.0a3 with simple FastAPI application with database, but at least partially covering most of the new features.
• Add a warning when overriding existing container (only relevant for string providers)
• Change the default of preserve_global_context in container_context() and DiContextMiddleware to true since a "global" context should not be so easy to reset.
• Fix documentation (some sections and especially enumerated lists do not show up correctly).
• Expand the documentation of some sections for clarity

[xelandernt]

@lesnik512 As mentioned above, I did a prelim test on a relatively simple application (similar to the example applications). I load tested it with ~150 RPS and did not have any issues.