Update deps and docs after urllib3 migration

[jku]

• Document RequestsFetcher as deprecated
• Make sure we mention that RequestsFetcher is still available (but deprecated, and needs an added dependency)
• Remove dependency on requests (and 3 indirect deps) after the code base no longer needs it (with the exception of the deprecated RequestFetcher)
• Document that TLS certs now come from operating system, point to truststore project as something applications may want to use

PRs handling this:

• replace RequestsFetcher for Urllib3Fetcher #2762
• More porting from requests to urllib3 #2773
• Handle proxy variables #2789

[jku]

#2773 (merged):

• Documents RequestsFetcher as deprecated
• Makes Updater not import RequestsFetcher by default
• Remove requests, idna, charset-normalizer, certifi from dependencies

#2789:

• Documents Updater default HTTP handling (urllib3, certificates, proxies)