docs: split plugin file into chapter files

[DjamilaMamedov]

The Plugin.rst file has been split into the chapterfiles: DirectorPlugins.rst.inc, FileDaemonPlugins.rst.inc and StorageDaemonPlugins.rst.inc .
The chapterfiles had been split into one file per plugin.

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[bruno-at-bareos]

I've made some suggestion to also fix all index syntax to use the sphinx one.
I think that mariadbplugin.rst can be remove as it serve nothing useful.

[bruno-at-bareos]

Suggested change

.. include:: Plugins/FileDaemonPlugins/MariaDBPlugin.rst.inc

There's nothing useful inside it, Corresponding file should be removed

[bruno-at-bareos]

This file need to be removed, nothing useful in.

[bruno-at-bareos]

There's still some needed changes, as I found some mispelled lower/upper case name in plugin file name.
The following files need to be renamed.
Problem with lower/upper case name of split plugin files

• docs/manuals/source/TasksAndConcepts/Plugins/FileDaemonPlugins/GlusterFsPlugin.rst.inc -> GlusterFSPlugin.rst.inc
• docs/manuals/source/TasksAndConcepts/Plugins/FileDaemonPlugins/LdapPlugin.rst.inc -> LDAPPlugin.rst.inc
• docs/manuals/source/TasksAndConcepts/Plugins/FileDaemonPlugins/MsSqlPlugin.rst.inc -> MSSqlPlugin.rst.inc
• docs/manuals/source/TasksAndConcepts/Plugins/FileDaemonPlugins/MySqlPlugin.rst.inc -> MySQL.rst.inc
• docs/manuals/source/TasksAndConcepts/Plugins/FileDaemonPlugins/PostgreSqlPlugin.rst.inc -> PostgreSQLPlugin.rst.inc

[bruno-at-bareos]

The build doc fail actually and that need to be fixed.