Re-organize quick start guide locations

[axelwalter]

Both user and contributors quick start guides were in the user tutorial section, accessible via a almost empty page that contained only two links.

Here, the quick start guides are moved to their appropriate locations.

Describe the change

PR checklist

• [ ] I have added description of the change I'm proposing in the OpenMS Documentation.
• [ ] I have read and followed OpenMS Documentation Contributing guidelines.
• [ ] I have attached a screenshot of the relevant area after this change.
• [ ] CHANGELOG.md is updated.
• [ ] I have added my name in CONTRIBUTING.md.

[timosachsenberg]

can you check why the test was failing?

[poshul]

/home/runner/work/OpenMS-docs/OpenMS-docs/docs/tutorials-and-quickstart-guides/openms-user-tutorial.md:55: WARNING: 'myst' cross-reference target not found: 'installations/installation-on-gnu-linux.md#build-openms-from-source' [myst.xref_missing]