Update Writing Extensions Page

WEBSITE-1078

Please review these changes @Nicofuma @DavidIQ @battye @michaelcullum @paul999 @Derky 
`http://phpbb-website:8888/extensions/writing/#a-template-events`

I think it might be useful to include internal links like these in more places. There are numerous instances throughout the article that can benefit from this (especially since the sidebar isn't visible when scrolling on mobile devices)