fix #3067: docs: add markdown primer from pve to pbs

this copies the markdown primer from the pve docs to allow access to
it via the help buttons in the gui

Signed-off-by: Stefan Sterz <s.sterz@proxmox.com>

docs/index.rst

@@ -50,6 +50,7 @@ in the section entitled "GNU Free Documentation License".
    file-formats.rst
    backup-protocol.rst
    calendarevents.rst
+   markdown-primer.rst
    glossary.rst
    GFDL.rst
 

docs/markdown-primer.rst

@@ -0,0 +1,178 @@
+.. _markdown-primer:
+
+Markdown Primer
+===============
+
+  "Markdown is a text-to-HTML conversion tool for web writers. Markdown allows
+  you to write using an easy-to-read, easy-to-write plain text format, then
+  convertit to structurally valid XHTML (or HTML)."
+
+  --  John Gruber, https://daringfireball.net/projects/markdown/
+
+
+The Proxmox Backup Server (PBS) web-interface has support for using Markdown to
+rendering rich text formatting in node and virtual guest notes.
+
+PBS supports CommonMark with most extensions of GFM (GitHub Flavoured Markdown),
+like tables or task-lists.
+
+.. _markdown_basics:
+
+Markdown Basics
+---------------
+
+Note that we only describe the basics here, please search the web for more
+extensive resources, for example on https://www.markdownguide.org/
+
+Headings
+~~~~~~~~
+
+.. code-block:: md
+
+  # This is a Heading h1
+  ## This is a Heading h2
+  ##### This is a Heading h5
+
+
+Emphasis
+~~~~~~~~
+
+Use ``*text*`` or ``_text_`` for emphasis.
+
+Use ``**text**`` or ``__text__`` for bold, heavy-weight text.
+
+Combinations are also possible, for example:
+
+.. code-block:: md
+
+  _You **can** combine them_
+
+
+Links
+~~~~~
+
+You can use automatic detection of links, for example,
+``https://forum.proxmox.com/`` would transform it into a clickable link.
+
+You can also control the link text, for example:
+
+.. code-block:: md
+
+  Now, [the part in brackets will be the link text](https://forum.proxmox.com/).
+
+Lists
+~~~~~
+
+Unordered Lists
+^^^^^^^^^^^^^^^
+
+Use ``*`` or ``-`` for unordered lists, for example:
+
+.. code-block:: md
+
+  * Item 1
+  * Item 2
+  * Item 2a
+  * Item 2b
+
+
+Adding an indentation can be used to created nested lists.
+
+Ordered Lists
+^^^^^^^^^^^^^
+
+.. code-block:: md
+
+  1. Item 1
+  1. Item 2
+  1. Item 3
+    1. Item 3a
+    1. Item 3b
+
+NOTE: The integer of ordered lists does not need to be correct, they will be numbered automatically.
+
+Task Lists
+^^^^^^^^^^
+
+Task list use a empty box ``[ ]`` for unfinished tasks and a box with an `X` for finished tasks.
+
+For example:
+
+
+.. code-block:: md
+
+  - [X] First task already done!
+  - [X] Second one too
+  - [ ] This one is still to-do
+  - [ ] So is this one
+
+Tables
+~~~~~~
+
+Tables use the pipe symbol ``|`` to separate columns, and ``-`` to separate the
+table header from the table body, in that separation one can also set the text
+alignment, making one column left-, center-, or right-aligned.
+
+
+.. code-block:: md
+
+  | Left columns  | Right columns |  Some  | More | Cols.| Centering Works Too
+  | ------------- |--------------:|--------|------|------|:------------------:|
+  | left foo      | right foo     | First  | Row  | Here | >center<           |
+  | left bar      | right bar     | Second | Row  | Here | 12345              |
+  | left baz      | right baz     | Third  | Row  | Here | Test               |
+  | left zab      | right zab     | Fourth | Row  | Here | ☁️☁️☁️              |
+  | left rab      | right rab     | And    | Last | Here | The End            |
+
+Note that you do not need to align the columns nicely with white space, but that makes
+editing tables easier.
+
+Block Quotes
+~~~~~~~~~~~~
+
+You can enter block quotes by prefixing a line with ``>``, similar as in plain-text emails.
+
+.. code-block:: md
+
+  > Markdown is a lightweight markup language with plain-text-formatting syntax,
+  > created in 2004 by John Gruber with Aaron Swartz.
+  >
+  >> Markdown is often used to format readme files, for writing messages in online discussion forums,
+  >> and to create rich text using a plain text editor.
+
+Code and Snippets
+~~~~~~~~~~~~~~~~~
+
+You can use backticks to avoid processing for a few word or paragraphs. That is useful for
+avoiding that a code or configuration hunk gets mistakenly interpreted as markdown.
+
+Inline code
+^^^^^^^^^^^
+
+Surrounding part of a line with single backticks allows to write code inline,
+for examples:
+
+.. code-block:: md
+
+  This hosts IP address is `10.0.0.1`.
+
+Whole blocks of code
+^^^^^^^^^^^^^^^^^^^^
+
+For code blocks spanning several lines you can use triple-backticks to start
+and end such a block, for example:
+
+.. code-block:: md
+
+  ```
+  # This is the network config I want to remember here
+  auto vmbr2
+  iface vmbr2 inet static
+          address 10.0.0.1/24
+          bridge-ports ens20
+          bridge-stp off
+          bridge-fd 0
+          bridge-vlan-aware yes
+          bridge-vids 2-4094
+
+  ```