Improve docs and migrate to separate website

[FabijanC]

Usage related changes

• Closes Documentation should be ported to a separate site #453
• Docs are mostly migrated intact from README.md to separate .md files under website/docs/
  • Some were improved (e.g. L1-L2 links were dead)
• Currently a new website will be deployed for each new push to main
• Pending:
  • examples section
  • documentation versioning

Development related changes

• Now the development process also depends on Node/npm
  • Updated the executor Docker image in config.yml
• Updated contribution guidelines with the content of the # Development section of README.md

Checklist:

• Checked out the contribution guidelines
• Applied formatting - ./scripts/format.sh
• No linter errors - ./scripts/clippy_check.sh
• No unused dependencies - ./scripts/check_unused_deps.sh
• No spelling errors - ./scripts/check_spelling.sh
• Performed code self-review
• Rebased to the latest commit of the target branch (or merged it into my branch)
• Updated the docs if needed, including the TODO section
• Linked the issues resolvable by this PR - linking info
• Updated the tests if needed; all passing - execution info