Update Writing Extensions Page

WEBSITE-1078
Refactor body class from controllers to template

WEBSITE-1158
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
Refactor template assignment to routing yml instead of controllers

For pages that don't require a controller function

WEBSITE-1158
Use StaticTemplateController for caching

WEBSITE-1158
Fix getInvolved route

WEBSITE-1158
Update deps
Merge pull request #43 from PayBas/task/refactor-header-images

Refactor body class from controllers to template
Merge remote-tracking branch 'public/master'
Fix EOF line for StaticTemplateController

WEBSITE-1158
Merge pull request #44 from PayBas/task/refactor-header-images

Fix EOF line for StaticTemplateController
More general corrections (Michael's notes)

WEBSITE-1078
Add links to core events and template events

WEBSITE-1078
Fix event list links

WEBSITE-1078
Preparing 3.1.2
Dont log event channel
Merge remote-tracking branch 'public/master'
Add to insta-deny policy about development extensions

WEBSITE-1078