[ticket/12568] Replace modifications explanation with extensions.

PHPBB3-12568
[ticket/12568] Update path to language pack installation page.

PHPBB3-12568
[ticket/12568] Update style installation instructions.

PHPBB3-12568
[ticket/12568] Remove various unnecessary references to phpBB3.

PHPBB3-12568
[ticket/12568] Fix typo - "directorie" -> "directories"

PHPBB3-12568
[ticket/12568] Update GitHub link.

PHPBB3-12568
[ticket/12568] The "Purge cache" option in the Customise tab no longer exists.

PHPBB3-12568
[ticket/12568] Fix awkward use of "phpBB Limited."

PHPBB3-12568
Merge pull request #2960 from prototech/ticket/12568

[ticket/12568] Update user docs