Altered the address region edit UI a little to improve clarity.
Also, close the hex dump viewer window when Escape is hit. (The
tool windows don't have "cancel" buttons, so the key has to be
handled explicitly.)
Added a warning to the index page that only appears when viewing it
over http/https. The issue is that the manual on the web reflects the
tip-of-tree sources, which may be inaccurate for whatever version the
user has.
Also, provide an alternate path to opening the manual in-app when
under development.
Move the SourceGen manual to a subdirectory in "docs", so that it can
be accessed directly from the 6502bench web site. The place where
it's installed in the distribution doesn't change.
