Landing pages for Community sections added

[PeterTurcan]

Landing pages for both the User Guide and Contributor Guide have been added to the Community sections, in order for there to be a page that can be linked to from the Learn section of the website. The landing pages include links to all the sections currently under the Community heading.

This is part of solving issue #244

[cppalliance-bot]

An automated preview of the documentation is available at https://247.site-docs.prtest2.cppalliance.org/contributor-guide/index.html