Add separate & improved how-to-contribute page

[dalito]

This pull request introduces several improvements to the documentation.

• Completely rewrote how-to-contribute.md with expanded step-by-step instructions, diagrams, and tabbed UI guidance
• Updated documentation build process to use uv package manager instead of pip
• Standardized spelling of "Skosmos" (previously "SKOSMOS") and "ZB MED" throughout documentation
• Added sphinxcontrib-mermaid extension for diagram support
• Improved release process documentation with clearer formatting
• Added multiple screenshots to guide contributors through GitHub web UI workflows

Open ToDos:

• Review and finish text of steps 5-7
• Add screenshots of GitHub Web UI where helpful (mainly step 5 + 7, see Add files via upload FranziFlecken/voc4cat#1)
• Add trouble shooting guide (e.g. recovering from diverging main)

Closes #152

[dalito]

I moved the contribution guide out of the guidelines into the page https://nfdi4cat.github.io/voc4cat/docs_usage/how-to-contribute.html and extended it. At some point we may drop the Guidelines (V2.0) / All-in-one-page (as published) since it can slowly replaced by the update HTML-guidelines and the How-to-contribute guide in this PR.

@nmoust - I merge this PR now. Since the changes are big you can better review of the rendered HTML page than the markdown with MyST markup formatting.

[nmoust]

@dalito: Everything looks really nice! I would only add in the "Create the pull request" section, that if the user cannot see "base repository: nfdi4cat/voc4cat base: main" option they should click on the link above with the text: "compare across forks". This might be the case if the contributor checks the PR from their own fork. The "compare across forks" text is not visible in the screenshot.

[dalito]

@nmoust thanks for the feedback.

What you refer to, is a PR created from the "Pull Requests" tab? If the repo has differs from upstream it is possible to start the PR directly via the main "Code" tab (maybe this is not so clear?).

[nmoust]

From my fork of Voc4Cat (synched), when I submit an xlsx file in the inbox folder, and click "Propose changes" (screenshot 1), the next screen gives me the option to merge my "test-branch" to my main (screenshot 2). But if I click the "compare across forks" link, I get the option to merge my "test-branch" to the NFDI4Cat/Voc4Cat main (screenshot 3).

[dalito]

I see. So it would be good to add a hint to click "compare across forks" if the user cannot select the upstream repo.