Improve import+formats docs and add explanations to upload UI

[philippotto]

This replaces #5400. I had to re-open the PR, since I updated the docs and the preview only works when the branch is prefixed with docs.

For the docs, I reworked the structure and text quite a bit. The new structure is roughly as follows:

• Managing Datasets
  • explains high-level ways of getting data into webknossos (e.g., uploading, file system). for non-wkw formats, it refers to Data Formats
  • explains concepts, such as editing dataset settings and dataset sharing
• Data Formats
  • explains Concepts (Layers, Magnifications etc)
  • explains wk formats (wkw + nml)
  • explains tools
    • conversion with webknossos
    • conversion with wkcuber

URL of deployed dev instance (used for testing):

• http://docsuploaduiformathint.webknossos.xyz/

Steps to test:

• read https://docs.webknossos.org/v/docs-upload-ui-format-hint/guides/datasets
• read https://docs.webknossos.org/v/docs-upload-ui-format-hint/reference/data_formats
• go to http://docsuploaduiformathint.webknossos.xyz/datasets/upload and check out new explanation texts/popovers (only available when jobsEnabled=true)

Issues:

• fixes Add a UI hint for supported formats #5399

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• Updated (unreleased) changelog
• Updated (unreleased) migration guide if applicable
• Updated documentation if applicable
• Adapted wk-connect if datastore API changes
• Needs datastore update after deployment
• Ready for review