Mixed additions, corrections and improvements

• Fixes a typo in the build instructions
• Corrects the communicated version in which Gradident Fill Layers are available to the upcoming 4.4.2 (incorrectly says 4.4 right now)
• Links a "go to IRC" step in the manual contributors guide to the instructions for joining IRC
• Adds an explicit section for "building the manual" to the manual contributors guide (reason being I was not able to find this information even using full-text-search across the whole repo with various terms)
• Points out the build step more clearly where it is currently documented (reason being I would have found it with Ctrl+F if it were labeled as such)
• Clarifies the convention of exactly when (and when not) to use | for preformatting text in the manual
• Adds an FAQ entry for the known issue of ampersand (&) characters appearing in docker titles
• Revises grammar and clarity of the Group Layers documentation page