Initial draft detailing how to create a new appeals page

[radishmouse]

Closes #728

• Updates the documentation about updates.thunderbird.net.
• Includes notes about creating/developing an appeals page

Still in progress: how to specify translated strings and how to address accessibility

[radishmouse]

@aaspinwall could you take a quick look at the draft? I'd love to get an idea of how this reads for someone who isn't familiar with the process. Thanks!

[radishmouse]

@MelissaAutumn Are there other topics I need to include?

[Sancus]

Please link this to #728

[aaspinwall]

Beautifully written as always! Great job

Loved how you mentioned that we deviate from the industry standard. Great to know

[MelissaAutumn]

Looks good, just a few nits/suggestions.