doc: Documentation fixups

This fixes things I found when translating the help for Balsa. There are more things needing updating, but they're a bit trickier, so I'll open a bug for those points. Changes in this commit:

• Fix dead links where I found the right new link.
• Fixups to typos and grammar, punctuation.
• Whitespace fixes by adding or removing them where appropriate.
• GUI strings changed to closer follow the GUI in cases where I found the right string.
• Use &Balsa; rather than balsa in strings where it was previously missed.

In this commit I have not touched balsa.xml, is that file still in use? I see it duplicates at least part of the contents of the .page files.

v2: Fix 3 more dead links. Remove two sentences about sending bugs through bug-buddy. It was removed from Debian in 2011, so is no longer around. The strings in question already mentions sending bugs through Gitlab.