Add additional install methods to quickstart

[raddevon]

Since the edgedb.com/install page is no longer linked prominently on the site, we should move the alternative method instructions that were there to this page, remove that page, and redirect it here.

[beerose]

Are we going to remove the /install page? If so, we can add a redirect to the docs/intro/quickstart page.

[raddevon]

Are we going to remove the /install page? If so, we can add a redirect to the docs/intro/quickstart page.

Yes, that's what I was thinking. I can merge this first, followed by a PR on edgedb.com to remove the old page and add a redirect.

[raddevon]

@beerose I created a PR to remove the /install page and redirect that page's URL to the installation section of the quickstart. Ideally, this should be merged before that one.