docs/ folder need work to change 3.1.x to 3.2.x in readme, install, changelog etc

Hi,

the .html-files in "docs/" need some work. In most files there is still the head with "3.1.x changelog"; "phpBB 3.1.x Install" etc.

But this should be change to "phpBB 3.2.x Install" etc.

Affects:
CHANGELOG.html
FAQ.html
INSTALL.html
README.html
coding-guidelines.html (only wording from ascareus to rhea)

Regards