Docs: Adding minor edits to the Quick Start Guide

[coder-karen]

Changes proposed in this Pull Request:

• Adding information about cloning using a public SSH key
• Minor edit to the first jetpack docker up prompt, to be jetpack docker up -d
• Changing the Jurassic Tube wording to indicate that you need to visit the homepage in order to create the subdomain

Jetpack product discussion

n/a

Does this pull request change what data or activity we track or use?

n/a

Testing instructions:

• Is it clear when reading through the doc that using a public SSH key is (or may be) the best choice if possible (but the HTTPS route also is still ok)? (Should we explain 'why' it may be recommended (more secure), or is that adding too much extra complexity here? Or are we already adding too much complexity by mentioning SSH now?)
• Also is it clear that visiting the homepage of Jurassic Tube is how to create a subdomain?

[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

• ✅ Include a description of your PR changes.
  
• ✅ All commits were linted before commit.
  
• ✅ Add a "[Status]" label (In Progress, Needs Team Review, ...).
  
• ✅ Add testing instructions.
  
• ✅ Specify whether this PR includes any changes to data or privacy.
  
• ✅ Add changelog entries to affected projects
  

This comment will be updated as you work on your PR and make changes. If you think that some of those checks are not needed for your PR, please explain why you think so. Thanks for cooperation 🤖

---

The e2e test report can be found here. Please note that it can take a few minutes after the e2e tests checks are complete for the report to be available.

---

Once your PR is ready for review, check one last time that all required checks (other than "Required review") appearing at the bottom of this PR are passing or skipped.
Then, add the "[Status] Needs Team review" label and ask someone from your team review the code.
Once you’ve done so, switch to the "[Status] Needs Review" label; someone from Jetpack Crew will then review this PR and merge it to be included in the next Jetpack release.

[sdixon194]

Thanks for these suggestions! Some thoughts on formatting but otherwise g2g 👍

[sdixon194]

Looks good, thanks for updating!