[MERGE] Add a brief Portability section to HACKING, build HACKING.htm from 'make docs'.

[Andrew Bennetts]

Martin Pool wrote:
> Martin Pool has voted +1.
> Status is now: Approved
> Comment:
> You could add the generated HACKING into index.html too...

Done.  (As a separate change because I'd already submitted the branch to PQM).

-Andrew.