Add small documentation details

Summary:
Add some small Diviner documentation details like:

* mention PHP 8.1 (thanks https://we.phorge.it/Q18)
* mention the fact that you need multiple databases (it may be not obvious from the startup)
* mention MariaDB and not just MySQL (thanks MariaDB community!)
* mention "Administrator" and not just "User" for the "Instructions for installing, configuring, and using Phorge."

I was not bold enough to do more changes.

Test Plan: I tested locally and Diviner does not look bad.

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: avivey, speck, tobiaswiese, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25064

src/docs/book/user.book

···
 {
   "name": "phorge",
-  "title": "Phorge User Documentation",
+  "title": "Phorge Administrator and User Documentation",
   "short": "User Docs",
   "preface": "Instructions for installing, configuring, and using Phorge.",
   "root": "../../../",

src/docs/user/configuration/configuration_guide.diviner

···
 The next steps are:
 
   - Configure your webserver (Apache, nginx, or lighttpd).
+  - Configure the databases.
   - Access Phorge with your browser.
   - Follow the instructions to complete setup.
 

src/docs/user/installation_guide.diviner

···
   - a domain name (like `phorge.example.com`);
   - basic sysadmin skills;
   - Apache, nginx, or another webserver;
-  - PHP, MySQL, and Git.
+  - PHP;
+  - MySQL (you will need a server with multiple databases);
+  - git
 
 The remainder of this document details these requirements.
+
+You may be interested also in preparing these optional stuff:
+
+  - have valid SMTP parameters for outgoing email notifications;
+  - having nothing listening on port 22, to then setup a SSH+git server
 
 Installation Requirements
 =========================
···
 You will also need:
 
   - **MySQL**: You need MySQL. We strongly recommend MySQL 5.5 or newer.
-  - **PHP**: You need PHP 5.5 or newer.
+    You will need a server with multiple databases.
+  - **PHP**: You need PHP 5.5 or newer. Note that PHP 8.1 and above are not
+    fully supported.
 
 You'll probably also need a **domain name**. In particular, you should read this
 note:
···
 
   - git (usually called "git" in package management systems)
   - Apache (usually "httpd" or "apache2") (or nginx)
-  - MySQL Server (usually "mysqld" or "mysql-server")
+  - MySQL Server (usually "mysqld" or "mysql-server" or "mariadb-server")
   - PHP (usually "php")
   - Required PHP extensions: mbstring, iconv, mysql (or mysqli), curl, pcntl
     (these might be something like "php-mysql" or "php5-mysqlnd")