Changes to 'add entry' form

[ots22]

(Mark as done when on #622)

• [x] Documentation general improvements
  • https://github.com/alan-turing-institute/scivision/issues/501
• [x] Prompt for login before completing form
• [x] Make 'Add new entry' button more prominent
  • added a 'contribute' button to the new catalog menu, and renamed 'add new entry' to 'contribute an entry'
• [x] Add additional tasks (see roadmap)
• [x] Clarify flow when there isn't a yaml file (new model form in particular - there is box quite far down the form, but several prompts for a link etc)
• [x] remove:
  • pretrained
  • format
• [x] some examples of form entries, good and bad (e.g. name of pip package, but don't need pip install ...)
• [x] store data in the form in case of navigating away from the page
• [x] don't make 'tags' required
• [x] include a message in the PR (see below)
• ~[ ] Don't redirect to GitHub after the PR is opened - make a page explaining what happened and link to it~
• [x] What if you don't have a GitHub account (link to resource elsewhere)
  • can sign up during the login flow - added a note to prerequisites
• [x] #618

Nice to have:

• ~[ ] pull tags in from GitHub~

[aldenc]

suggested text for new PR generated by Scivision frontend:

This is an automated message from the Scivision frontend:

"Your submission was successful and will be reviewed by the Scivision team and added to the catalog. If we have any questions about your submission we will post them as a comment in this PR."

[aldenc]

change format requirement under prerequisites