Consider implementing an index file that sets the contents of TOC

[stanislaw]

@mettta: let's capture this to not forget. It's nice to be able to customize the structure of TOC.

Example: @elfman2's DO-178C TOC on Read The Docs is nice:

Plans:
    [1. Plan for Software Aspect Of Certification](https://strictdoc-templates.readthedocs.io/en/latest/rst/software/plans/PSAC.html)
...
Standards:
    [1. Software Code Standard](https://strictdoc-templates.readthedocs.io/en/latest/rst/software/standard/SCS.html)
...
Requirements:
    [1. System Requirements](https://strictdoc-templates.readthedocs.io/en/latest/rst/system/SR.html)
...
Verification:
...