website build instructions

[Graham Percival]

I've just added this to the CG website chapter:

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Build the website with:

@example
make website
@end example

@noindent
however, please note that this command is not designed for being
run multiple times.  If you see unexpected output (mainly the page
footers getting all messed up), then delete your
@file{out-website} directory and run @code{make website} again.
--------

If this doesn't work for you, let me know and we'll figure out
what's happening.

Cheers,
- Graham