docs: refactor title and organisation

[mloiseleur]

Description

1. Reduce title length in order to get a readable left menu.
2. Move tutorials related to sources to sources section, like the others
3. Fix some markdown formatting issues
4. Removes an invalid field in mkdocs configuration: trademark
5. Fixes Minikube externaldns + coredns documentation is out of date #4723 by warning user.

Current left menu:

With this PR, the left menu:

Checklist

• End user documentation updated

[szuecs]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: szuecs

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [szuecs]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[mloiseleur]

@szuecs I can self approve but I cannot self lgtm
/lgtm

[k8s-ci-robot]

@mloiseleur: you cannot LGTM your own PR.

In response to this:

@szuecs I can self approve but I cannot self lgtm
/lgtm

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[Raffo]

/lgtm