feature: recursive nesting for deeper levels in side-nav

[ntno]

Description

optionally allows an increase in navigation nesting in the side navigation panel. previously only 2 levels of nesting were supported. this is beneficial because it means a user can have deeper documentation organization.

Example

previous to this change only the "Release Notes" index page would appear in the side navigation.

├── docs
│   ├── about
│   │   ├── contributing.md
│   │   ├── index.md
│   │   ├── license.md
│   │   └── release-notes
│   │       ├── index.md
│   │       ├── v0
│   │       │   └── notes.md
│   │       ├── v1
│   │       │   ├── index.md
│   │       │   └── notes.md
│   │       ├── v2
│   │       │   ├── index.md
│   │       │   └── notes.md
│   │       └── v3-alpha.md
│   ├── help.md
│   ├── index.md
│   ├── macros-test.md
│   ├── section-index-notes.md
│   └── user-guide
│       ├── configuration.md
│       ├── custom-themes.md
│       ├── deploying-your-docs.md
│       ├── styling-your-docs.md
│       └── writing-your-docs.md
├── mkdocs.yml

Describe the purpose of this PR along with any background information and the impacts of the proposed change. For the benefit of the community, please do not assume prior context.

Provide details that support your chosen implementation, including: breaking changes, alternatives considered, changes to the API, etc.

increase the nesting using the mkdocs extra attribute:

extra:
  MAX_SIDE_NAV_SECTION_NESTING: 3

demo mkdocs.yml

References

Include any links supporting this change such as a:

• GitHub Issue/PR number addressed or fixed
• Related pull requests/issues from other repos

If there are no references, simply delete this section.

Testing

Describe how this can be tested by reviewers. Be specific about anything not tested and reasons why. If there are existing unit and/or integration test suites, tests should be added for new functionality and existing tests should complete without errors.

Please include any manual steps for testing end-to-end or functionality not covered by unit/integration tests.

Also include details of the environment this PR was developed in (language/platform/browser version).

• This change adds test coverage for new/changed/fixed functionality

Checklist

• I have added documentation for new/changed functionality in this PR or in mkdocs-terminal/documentation
• All active GitHub checks for tests, formatting, and security are passing

[ntno]

TODO: probably makes sense to update the extra variable name to be prefixed with TERMINAL_ (or similar)

moved to theme attribute