Update formatting of documentation part

This uses the 'always one space' format used in expl3, and therefore
includes an adjustment to the demo environment code to gobble an
additional space.
Update/fix mark up for LaTeX, BibTeX, etc.

There were rather a lot of 'plain text' uses, which are now fixed. Also,
I've concluded that `\LaTeX{}`, _etc._, is usually better than
`\LaTeX\ `.
Sort out copyright years
Add section lead-in for code-level documentation
Document \notestobib_write_note:nn

This is renamed from \@@_write_note:nn, leading to a few internal
changes.
Merge branch 'master' of github.com:josephwright/notes2bib