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

Make D9 support visible

See merge request documentation/docs/installation!26

This option was often set permissively in D8 project-templates - and it's
often set restrictively in D9 project-templates.

Adding the step generally shouldn't change anything for D8 admins -- and for
D9 admins it enables installation to proceed.

* This is predictably getting out of sync 'docs/drupal8'
* 'docs/drupal8' is getting updates to reference both 8+9

Update requirements.md  based on https://civicrm.org/blog/dev-team/civicrm-534-release

See merge request documentation/docs/installation!25

Move the multisite installation from the SysAdmin guide

See merge request documentation/docs/installation!22

Timestamp mismatch warnings link to this doc - make it more useful.

See merge request documentation/docs/installation!23

Add historical PHP & MySQL compatibility info

See merge request documentation/docs/installation!24

Remove the section on clean URLs in WordPress

Closes #7

See merge request documentation/docs/installation!20

This is being migrated to the SysAdmin guide as it is no longer required
for new installations

Docs for how to use unicode fonts for pdf output

See merge request documentation/docs/installation!19

Change some broken absolute links to relative links

See merge request documentation/docs/installation!18

documentation/docs/sysadmin#3 - Rework "Get the code" and "Run the installer"

See merge request documentation/docs/installation!15

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.

* Use simpler language
* For optional commands, link to relevants docs.

* There are a lot of details that can live inside collapsed sections.
* Several of the asides are basic gotchas that would affect any effort to
  run composer on D8. Combine these into one collapsed section,
  "Quick and dirty introduction to D8 with composer"
* The headings "Expert mode"/"Guided mode" are weird to me -- because
  (1) "Expert" identifies the most common/easiest variation of the command,
  and (2) "mode" suggests software-mode, but it's actually a different
  presentational style for the documenter. Instead, just give one
  procedure - and provide drill-down options for explanation.

* 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.