[doc] explain how to do postprocessing for literate forester

[dannypsnl]

resolve #7884

[andreasabel]

Rendering at: https://agda--7886.org.readthedocs.build/en/7886/tools/literate-programming.html#literate-forester

[andreasabel]

Thanks @dannypsnl ! I pushed some fixes to the documentation (format/typos).
I'd like to test the instructions before publishing, but I am blocked on:

• Build failure due to unstable feature use dannypsnl/agda-tree#10

[dannypsnl]

@andreasabel Please check if you can follow https://github.com/dannypsnl/agda-tree-example to get highlight.