[docs-only] add toc to changelog

[mmattel]

Description

Adds table of contents to the top of the CHANGELOG.md file.

Anchor links in markdown are generated from the content of the header according to the following rules:

1. All text is converted to lowercase.
2. All non-word text (e.g., punctuation, HTML) is removed.
3. All spaces are converted to hyphens.
4. Two or more hyphens in a row are converted to one.
5. If a header with the same ID has already been generated, a unique incrementing number is appended, starting at 1.
6. All changelogs for 2.x and below have a main entrance link

Related Issue

• Relates to Improve and harmonize changelogs in the source repos docs#4901
• Changes copied from: [docs-only] add toc to changelog ocis#7691

[michaelstingl]

@mmattel could you post the desktop TOC to get an idea?

[mmattel]

Run locally, works nicely 😄

[fmoc]

Will need to test this locally.

[fmoc]

See above.

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

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[mmattel]

Also include the earlier versions or how to run locally...

owncloud/ios-app#1287 (comment)