Update Writing Extensions Page

WEBSITE-1078
Notes from PayBas

WEBSITE-1078
Promote usage of path instead of pattern in routing.yml

WEBSITE-1078
Add some additional Migration wiki links

WEBSITE-1078
More general corrections (Michael's notes)

WEBSITE-1078
Add links to core events and template events

WEBSITE-1078
Fix event list links

WEBSITE-1078
Add to insta-deny policy about development extensions

WEBSITE-1078
Fix Extension Quickstart alt tags

WEBSITE-1078
Merge pull request #42 from VSEphpbb/task/extension-writing

Update Writing Extensions Page