docs: add standalone Contributor License Agreement page (refs #12863)#13225

AshrafHosam wants to merge 3 commits intoEbookFoundation:mainEbookFoundation/free-programming-books:mainfrom

AshrafHosam:fix/standalone-cla-pageAshrafHosam/free-programming-books:fix/standalone-cla-pageCopy head branch name to clipboard