WIP: Document tab completions

This PR adds a new chapter to the manual listing all currently defined tab completions for Unicode input.

The table is generated by a script that loads in `base/latex_symbols.jl` and spits out its contents to a ReST table. 

To-do
- [ ] Where should this go? It really feels like this should be an appendix, but I'm not sure what the best layout should look like.
- [ ] `make latexpdf` is very unhappy at the large number of Unicode characters pdflatex has to include.