Trigger chapter review

Made the changes discussed during the last Trigger Core software meeting (https://indico.cern.ch/event/1540634/#114-documentation-update):

• Added new section "Infrastructure and tools" with its landing page
• Added new section "Analysis tools" with its landing page
• Added landing page under Trigger as introduction
• Added landing page as introduction in Menu for expert section
• Added new chapter athena/trigger/infrastructure/ttweb.md
• Moved triggerdb.md from menu to infrastructure
• Moved menutriggerapi.mp from menu to analysis
• Moved all the Trigger files in different sections subfolders
• Fixed internal links