docs: remove 'Additional Notes' from TOC, fix markup
docs: remove 'Additional Notes' from TOC, fix markup

What's covered in the 'Additional Notes' section of the documentation
hardly seems important enough to deserve its own entry in the sidebar.

mhubig/borg,raxenak/borg,mhubig/borg,raxenak/borg,edgimar/borg,RonnyPfannschmidt/borg,edgewood/borg,raxenak/borg,edgimar/borg,RonnyPfannschmidt/borg,edgimar/borg,edgimar/borg,RonnyPfannschmidt/borg,edgewood/borg,RonnyPfannschmidt/borg,RonnyPfannschmidt/borg,mhubig/borg,edgewood/borg,raxenak/borg,edgewood/borg