Fixes #11

I left the other part until we actually cut 5.40 due to the framing

CIVICRM-1707 Remove incorrect statement about open_basedir being deprecated. Safe mode was deprecated in PHP 5.3, so no longer relevant. Include minimum PHP configuration recommendations and links.

Update Requirements doc to mention that PHP7.4 is supported from MySQL 5.7 and also to clarify that all CMSes now supoort MySQL8 and add a note about installing on Cloud Service Providers such as AWS and Azure

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.

* As in the previous commit (D7/BD), there are two workflows, web-based an CLI-based.
  Put them in collapsible admonitions.
* Reorganize the warnings for the web-based workflow.
* The D8 section had more discussion of the CLI steps. Move these over to the `cli-cv.md` file.
  They'll still need more editing, but it makes sense for this more advanced+verbose
  use-case to have a separate file.

The list of requirements is hard to take in at a glance. This adds a section that
you can read at a glance. It's obviously not as thorough as the main requirements,
so it's only presented as a summary-recommendation.

This repo currently has 16 markdown files, of which 8 have `CR` endings and
8 have `CR-LF` endings.  For comparison, the dev-docs and sysadmin-guide
have strictly `CR` endings.