docs(guides): moved simplecache from docs

ref: #6812
docs(faq): moved What should I use to edit php code from docs

ref: #6812
docs(admin): moved backup and restore page from docs

ref: #6812
docs(guides): moved the gatekeeper page from docs

ref: #6812
docs(faq): moved the changing profile fields page from docs

ref: #6812
docs(faq): moved the Changing registration page from docs

ref: #6812
docs(guides): move plugin coding guidelines from docs

ref: #6812
docs(faq): move the I don't like the wording of something in Elgg page
from docs

ref: #6812