Merge "For Developers" page into the "Community" page

[PrasadBabarendaGamage]

The "For developer page" does not seem very useful as the majority of the links point to the community page. To simplify navigation, for now Andy, and I think it best to:

• merge "For Developers" into Community
• move "Extending OpenCMISS" from Documentation to Community
• remove Contributors section from Community and replace it with a Developers section

[maierbn]

merge "For Developers" into Community

• We could completely remove the "For Developers" page as it only contains links.
• Instead collect the links on the "Contributing" page

move "Extending OpenCMISS" from Documentation to Community

• Yes, this page is currently called "Contributing to OpenCMISS" and should be a child page of "Community"

remove Contributors section from Community and replace it with a Developers section

• Also move "GitHub issues" section from Community to "Contributing" page