docs: Backport multiple changes from v2-docs into V1 docs.

[microbit-carlos]

These have been manually added, as the commits with these changes in the v2-docs branch contain a lot more V2-specific information.

https://github.com/bbcmicrobit/micropython/compare/v2-docs

I tried to separate this into 4 commits, one for the "Hex format" page, one for the the "REPL" page, one for the "IO" pages and the last one captures a few small changes from several files.

Review URL: https://microbit-micropython--741.org.readthedocs.build/en/741/

[microbit-carlos]

@microbit-mark could you review once you get a chance? These are mostly copy/paste changes from the v2-docs branch.

[microbit-mark]

All LGTM thanks @microbit-carlos