Add documentation about eta/phi heat map

[EdwardMoyse]

See #876

[Abish-13]

Hi @rx18-eng , I see you are working on this! I'm interested in helping out with the documentation for the eta/phi heat map. Would you like a hand drafting the content, or is there a specific part you'd like me to focus on?

[rx18-eng]

Hey @Abish-13 feel free to work on this , rasie a pr and i will review it !