Document t8code's folder structure with README.md files.

[holke]

Feature request

Is your feature request related to a problem? Please describe.
Our folder structure is not well documented which makes it hard for new developers to get into t8code quickly.

Describe the solution or feature you'd like
Each folder and subfolder containing source code should have its own README.md file, similar to https://github.com/DLR-AMR/t8code/tree/main/src/README.md and https://github.com/DLR-AMR/t8code/tree/main/src/t8_cmesh/README.md

At least these main folders must be covered:

• /test
• /example
• /src (main folder already in Documentation: README files /src and /src/t8_cmesh #1855 but subfolders still missing)
• /tutorials
• /benchmarks
• /scripts

And all of their subfolders.

If you want to add a new README.md file, please add a subissue to this issue - see #1873 as example.

The README.md files should cover at least:

• What subfolders are in this folder. Brief description and link to their respective README.md files.
• What are the main features in this folder and in which files can they be found.
• What are the main entry points for developers in this folder.
• Links to related Wiki articles.
• Links to related code examples.

At best, each README.md file gives a brief description of each source/header file in the folder.

Describe alternatives you've considered
Not documenting is not an alternative

Estimated priority

"Priority: medium" Should be solved within half a year

Additional context

[Davknapp]

Can you create subissues out of the todo-boxes? it provides a much better overview of the progress.