[ADD] events: added complete documentation

[ksc-odoo]

Task ID: 2941079

There was no documentation for the Events app, so I created documentation for v14 (which can also be directly applied to v15).

[robodoo]

Pull request status dashboard.

[ksc-odoo]

Hey @mivu-odoo

There was no documentation for Events whatsoever in v14 or v15. Can you please review the content to make sure everything looks okay? EBJ has already looked over the content for accuracy.

Once I get your approval, I'll forward it over to Zac.

Thanks!

[samueljlieber]

Hey @mivu-odoo and @ksc-odoo!

I fixed the build errors - just so you are aware, here is what was causing the errors 🙂

1. events.rst was not included in the marketing.rst .. toctree::
2. Image :align: & :alt: tags were not properly indented underneath the .. image:: tag
3. There were a few trailing spaces left throughout the documents
4. On line 17 of selling_tickets.rst there was an internal reference to a document that doesn't exist
5. A few :guilabel: tags weren't formatted correctly