Explain the structure of directories and (index) RST files

[markkuriekkinen]

Ensure that one of the first RST chapters clearly explains the structure of the course module directories as well as the root and module index RST files. It is not enough that the manual course has its own module directories and index RST files as examples without explaing them in writing.

[markkuriekkinen]

Toctree explained here:
https://github.com/apluslms/aplus-manual/blob/fe6155ea1bf38d87b81f7225d5bd3e27cce4b69f/rst_guide/advanced_syntax.rst