Add docs for memoization

[sfalmo]

Add documentation in FAQ as discussed in #131.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.34%. Comparing base (2e377d5) to head (bfd5414).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #132 +/- ## ======================================= Coverage 97.34% 97.34% ======================================= Files 9 9 Lines 188 188 ======================================= Hits 183 183 Misses 5 5 ```

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[sfalmo]

How funny, v0.15.11 of ComponentArrays.jl, which was released 2h ago, now provides an rrule that makes Zygote work. Hence this @test_broken check actually passes. I think we should address this in a new PR.

[gdalle]

Wow, that's fun indeed ^^ wanna do the honors?

[gdalle]

Ok we're good, I also did a bit of cleanup. Wanna merge main into this?

[gdalle]

The diff is huge now, can you merge main into this branch to highlight only your changes to the docs?

[sfalmo]

Sorry, did some weird rebasing by accident. Now it should be fine.