Add guidelines for writing HLT2 lines

Adds a new page to the documentation with advice on tuning rates, bandwidth, and timing. Preview available here. I also moved around a few sections in the table of contents to sort by trigger stage rather than reconstruction/selection.

Tries to address #142 (closed) and #217 (closed). It's very verbose, I'm not sure if that suits the target audience (line writers). There are also several holes based on our inability to measure certain things very well/automatically. It's a start at least.

/cc @sstahl @gligorov @rmatev @mvesteri for feedback

/cc @abertoli @shunan for comments on how much this addresses your questions