[Doxygen] Include github markdown file for code guidelines

[enen92]

Description

Includes the markdown file we have on github also on doxygen.

Motivation and Context

Right now we have two files in different formats for the same content. Code guidelines were updated recently on github but not on doxygen. There are some issues with the solution:

• Links in the table of contents do not work
• Some strange characters at the beginning of the page (already fixed in the current doxygen version)

Maybe something may be worked in the future to solve the issues in doxygen. For now, better than having incorrect duplicated content.

How Has This Been Tested?

compile docs and see. Check codedocs

Screenshots (if appropriate):

You can see the end result here: https://codedocs.xyz/enen92/xbmc/md_docs__c_o_d_e__g_u_i_d_e_l_i_n_e_s.html

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• None of the above (please explain below)

[AlwinEsch]

That's very good for me to have only one place, and if the Markdown support in doxygen is now better +1 :-). With the same things in different places brings otherwise always deviations.

I hope with the links becomes fixed soon by doxygen (otherwise we become issue's :-D )

[yol]

Thanks a bunch! 👍