Missing modules in the docs

The following MicroPython modules are not referenced in the docs:

* array
- [Upstream docs](https://docs.micropython.org/en/latest/library/array.html)
* builtins
- [Upstream docs](https://docs.micropython.org/en/latest/library/builtins.html)
* ucollections
- [Upstream docs](https://docs.micropython.org/en/latest/library/ucollections.html)
* gc
- [Upstream docs](https://docs.micropython.org/en/latest/library/gc.html)
* math
- [Upstream docs](https://docs.micropython.org/en/latest/library/math.html)
* ustruct
- [Upstream docs](https://docs.micropython.org/en/latest/library/ustruct.html)
- Related issue: https://github.com/bbcmicrobit/micropython/issues/652
* sys
- [Upstream docs](https://docs.micropython.org/en/latest/library/sys.html)

We should either port the upstream docs (note that the module might only be partially enabled in the micro:bit port) or add an entry and link to the upstream docs.
Taking in consideration we have already ported `utime`, `micropython`, and `machine`, we should probably do the same for these.

PRs are welcomed and I'd be happy to walk through the process with anybody that might want to contribute to the project :)