[ADD] events: event tracks (v16)

[ksc-odoo]

This PR is adding a doc about event tracks to Odoo 16. (It is, essentially, a manual backport of the content that is currently on 17). At the moment, there is zero documentation for 16.0 Events users, who are curious about event tracks, and since the 17.0 version was recently reviewed and merged, I figured it was wise to add this information to this version, but with all the version-specific adjustments made throughout.

• Accurate flow

• Version-specific screenshots

PROJECT TASK: https://www.odoo.com/odoo/project/3835/tasks/4106040

[robodoo]

[ksc-odoo]

Hey @Felicious -- this is ready for a Peer Review from you, whenever you get a chance. Thanks! 👍

[ksc-odoo]

Thanks for the always-helpful feedback, @Felicious -- I made all those suggested edits and did a self-review of this PR to tighten up the overall content to make it as concise/direct as possible. I believe this one is ready for a Final Review from you, @StraubCreative (whenever you get a chance). Thanks! 👍

[StraubCreative]

@fw-bot up to saas-16.4

[samueljlieber]

@robodoo fw=no

[robodoo]

Disabled forward-porting.

[robodoo]

@ksc-odoo @samueljlieber unable to stage: merge conflict

[Xavier-Do]

@robodoo r-

[samueljlieber]

Resolved merge conflict in https://github.com/odoo/documentation/commit/9fdf74bc9e4b68a7647aa54cd460adc97a2920c1

[samueljlieber]

@robodoo r+