There are other expansions available, but I don't really think they make
sense, or why there are two of them for the same thing (`$FILE` and `%`
expand to the file name).
I added `$f` and `$p`, which expand to the document's path and current
page respectively, while keeping the other 2 just in case it breaks
someone's setup. I also documented them in both `zathura(1)` and
`zathurarc(5)`.
- in zathura.1.rst, move "Environmental variables" section closer to bottom
- in zathurarc.5.rst, change from using subsections to using
definitions (in order to make levels clearer on output to terminal)
