Update Writing Extensions Page

WEBSITE-1078

Please review these changes @Nicofuma @DavidIQ @battye @michaelcullum @paul999 @Derky 
`<div class="note-box blue"><strong>Note:</strong> The phpBB 3.1 coding standard states...`
Could "coding standard" link to https://area51.phpbb.com/docs/31x/coding-guidelines.html ?