docs: Branch with the V2 documentation

[microbit-carlos]

This draft PR shows the doc updates for micro:bit V2, which is deployed to ReadTheDocs as the default entry point, which expands to: https://microbit-micropython.readthedocs.io/en/v2-docs/

Many of the changes to the v2-docs branch that were not specific to V2 have been backported to the V1 docs, and this branch will be constantly rebased.

Having this as opened as PR makes GitHub track the changes between rebases (it adds a timeline entry linking to the old tree), so it can be used for traceability in case some merge/rebase goes wrong.

This PR surseeds https://github.com/bbcmicrobit/micropython/pull/743, which was accidentally opened from a branch in my fork instead of a branch in this repo.

[microbit-carlos]

Adding a comment here to ensure GitHub doesn't remove the entry where the last force push moved from https://github.com/bbcmicrobit/micropython/commit/b47d7e9e8f963e6a4ac7072af67144423cce5009 to https://github.com/bbcmicrobit/micropython/commit/2a38b7ad9c8064f9c94a0fd8a13734ec2ca26c1b