Add documentation for link decorators.

[msamsel]

Suggested merge commit message (convention)

Docs: Documentation for link's decorators. Closes ckeditor/ckeditor5#4848.

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Additional information

⚠️ branch target to t/186 to have available feature to build snippets.

[coveralls]

Coverage remained the same at 100.0% when pulling 206c753 on t/229 into 40d8266 on master.

[oleq]

The documentation is out-of-sync with #222 and needs to be updated first.

[msamsel]

@oleq I've updated the references to decorator features. So you can take a look on this guide and provide some feedback about it.
I also change target branch to be master, because link decorators are merged.