Add special hub boot routines to docs.

[gpdaniels]

  I found the following in my notes (it has been a while), but it would be good if someone wants to confirm:

• Hold left during boot: Do not mount file system, so skip executing boot.py and main.py.
• Hold right during boot: Mount file system, run boot.py but skip main.py.
• Hold left + right: Mount, but then reset the file system, creating a new boot.py and main.py.

I don't know if left+right will work if the file system is already corrupted. If it's the same as the Pyboard reset then it should probably work.

And if not, you should still be able to hold only left, and then use the REPL to reset the filesystem instead.

Originally posted by @laurensvalk in #41 (reply in thread)

[laurensvalk]

It turns out that the Left+Right combination does not work.

I made this page for that purpose instead: https://dfu.pybricks.com/fix-update