doc: disable root toc entry

[tgilon]

Closes # (if applicable).

Changes proposed in this Pull Request

This PR suggests to disable root TOC entries in order to declutter the table of contents for the rules overview.

Master

Current PR

Checklist

Required:

• Changes are tested locally and behave as expected.
• Code and workflow changes are documented.
• A release note entry is added to doc/release_notes.md.

If applicable:

• Changes in configuration options are reflected in scripts/lib/validation.
• For new data sources or versions, these instructions have been followed.
• New rules are documented in the appropriate doc/*.md files.

[fneum]

Fine, I'd like to split this into more sections later.

[tgilon]

Fine, I'd like to split this into more sections later.

@fneum Would make sense, I've myself introduced sections for the Open-TYNDP rules here: open-energy-transition#761