General themes/changes:

* Rewrite as a standalone page. This was previously content in the D8 page.
* Include the `curl ... cv.phar ...` command.
* Move discussion of the `core:install` options and common problems into collapsed admonitions.
* The "next level" docs for `cv core:install` are in the Dev Guide. Add links.
* Add a section for "Configure the site".
* Add a "Limitations" section.