Initial implementation of Subscription Docs

[engylemure]

Adds a advanced topic for Subscriptions in the book and a small example for it without the usage of a web integration. This PR fixes #588.

PR Checklist

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[LegNeato]

These look awesome! @nWacky any interest in making sure it matches what you were thinking?

[nWacky]

Hello. Thank you so much for adding documentation on subscriptions, that looks great.

I'd dive a bit deeper in some topics to help to familiarize people with subscriptions, but the documentation is good even without that

[engylemure]

@nWacky I have added these requested changes into the documentation, thanks for the review.

[LegNeato]

Approved with some small edits. Thank you so much! We can clean up the rest / fix the links after it lands.