website: add a 'watch' target
website: add a 'watch' target

The 'make watch' uses 'entr' to trigger do trigger a 'make build' if
any of the source files changed.

'entr' can be installed on Debian/Ubuntu using 'apt-get install entr'
and 'brew install entr' on OS X.

Change-Id: I926cb9769a9bd74312ba0f5a1b5e0c12bb1a84cc

vanadium/website,vanadium/website,vanadium/website