[Getting started] Set env variables on Windows using dialog

The instructions currently suggest using setx to set environment
variables on Windows. However, this truncates the variable at
1024 characters, which may cause the Path in particular to be
set incorrectly.

Use the env variables dialog instead.

Also add a note about setting ANT_HOME.
[Getting started] Advise setting JAVA_HOME on Linux
Add feedback page
Add 'report bugs' item to both contribute and documentation
[feedback] Style/layout/wording tweaks
Pre-populate bug report form description