First pass at custom effect docs.

Copied original content from wiki https://github.com/mono/MonoGame/wiki/Effects-And-Shaders.
Updated the docs a bit.
Update custom_effects.md
Update custom_effects.md
First pass at content intro section.
Update content_intro.md
Merge branch 'develop' into doc2
Added the start of the content section.
Merge branch 'doc2' of github.com:SickheadGames/MonoGame into doc2
Merge pull request #2568 from SickheadGames/doc2

Adding Content Docs