Fix documentation sections that still refer to u* modules, like ure and ujson

[dhalbert]

Some native modules that we've renamed back to their CPython names are still referred to with their u* names in the readthedocs documentation, e.g. circuitpython/docs/library/ujson.rst.

• Rename these files and also fix the module names mentioned inside them.
• Remove caveat language about these modules not being supported or that they may change.