Update guessing docs

[lilyminium]

When MDAnalysis/mdanalysis#3753 is merged we'll need to update the guessing tutorials here.

[aya9aladdin]

I started working on the user guide here: #213
this was a long time ago though so I'll revise it to add updates

[orbeckst]

@aya9aladdin , apparently, a number of people are really asking for more docs on the guessers so it would be great to have them — maybe you can also use your blog post(s) to update PR #213 ?