doc: apply general documentation updates - February 2021

[brodycj]

(updated)

for upcoming 0.20.0 release (see issue #409)

• React Native pre-0.63 support is now deprecated (see issue BREAKING CHANGES coming in 0.21 #423)
• known issue with React Native 0.64 (rc) on iOS (see issue Example app build issues with Flipper on iOS #422)
• add link to @otobank fork (see RFC: capture updates from OTOBank fork #425)
• document --use-apple-networking as EXPERIMENTAL & NOT SUPPORTED
  (see issue --use-apple-networking issue in generated example #426)

[brodycj]

Merging now to include the OTOBank fork discussed in issue #425

/cc @keima