Update docs for truth particles

As @svanstro pointed out, the overlap removal got a bit more complicated when I rewrote the truth handling in !322 (merged). I added some documentation to hopefully make that a bit easier to understand.

I also changed the field selection in the truth blocks to association since it's only used when associating the particles to jets.

While I was there, I thought I'd add mermaid as a dependency in mkdocs building, since flowcharts are nice (and gitlab and codimd both support mermaid). This prompted me to also add a docs/requirements.txt file, since the list of things we need is getting a bit longer these days.