docs(admin): moved duplicate installation from docs

ref: #6812
docs(faq): moved How do I find the code that does x from docs

ref: #6812
docs(guides): moved Engine/Controllers/BestPractices from docs

ref: #6812
docs(admin): moved the getting help page from docs

ref: #6812
docs(faq): moved how does registration work page from docs

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

ref: #6812
docs(guides): moved don't modify core page from docs

ref: #6812
docs(faq): moved How do I change PHP settings using .htaccess from docs

ref: #6812
docs(guides): moved hmac authentication page from docs

ref: #6812
docs(faq): moved Manually add user from docs

ref: #6812
docs(plugins): moved the diagnostics page from docs

ref: #6812
docs(faq): split the faq page into different files for readability

ref: #6812
docs(faq): moved the debug mode page from docs

ref: #6812
docs(admin): moved the plugin order page from docs

ref: #6812