Promote Quickstart in nav bar

[melpadden]

What does this PR fix/introduce?

Promote the Quickstart link to be the first thing a user sees.
Make it open in the same window
Make it a relative Docusaurus link

• Docs are successfully building - yarn install && yarn run build.
• For new internal links I used relative file paths (with .md extension) - e.g. ../../faq/faq-general.md - instead of introducing absolute file path, or relative/absolute URL.
• All external links have been verified with yarn run check:externals.
• My changes follow the Casper docs style guidelines.
• All technical procedures have been tested (if you want help with this, mention it in Reviewers).
• If structural changes are introduced (not just content changes), cross-broswer testing has been completed.

Reviewers

// Tag whoever needs to review. If you're not sure, leave blank.
@ACStoneCL

[github-actions]

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