Docs: What’s new & Upgrade guide 10.4 (#83133)

* Updated index pages and added v10.4 breaking changes, upgrade guide, and what's new pages

* Added what's new entries

* Removed empty headings

* Fixed link

* Removed duplicate entry and fixed styling

* Removed breaking changes page and references to it

* Added intro text

* Docs: 10.4 technical note for alertingUpgradeDryrunOnStart (#83262)

* Docs: 10.4 technical note for alertingUpgradeDryrunOnStart

* Apply suggestions from code review

Co-authored-by: Isabel <76437239+imatwawana@users.noreply.github.com>

* Address PR comments

* restarts -> starts

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Co-authored-by: Isabel <76437239+imatwawana@users.noreply.github.com>

* Fixed spelling

* Added new entries to What's new

* reorder and add intro

* Added PagerDuty data source entry

* Added entries

* Added new entries

* Fixed formatting

* Fixed page weight and links

* Apply suggestion from review

Co-authored-by: Mitch Seaman <mjseaman@users.noreply.github.com>

* remove team lbac, move return to previous (#83921)

- Remove Team LBAC for Loki (it is Cloud only)
- Move Return to Previous into the Alerting section, since it only works for Alerting now
- Add a note that data sources are separate from Grafana but included for visibility

* Replaced manual note with admonition shortcode

* move Return to Previous out of Alerting section

* Added youtube links

* Commented out youtube videos and removed duplicate video embed

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Co-authored-by: Matthew Jacobson <matthew.jacobson@grafana.com>
Co-authored-by: Mitchel Seaman <mitchel.seaman@gmail.com>
Co-authored-by: Mitch Seaman <mjseaman@users.noreply.github.com>

docs/sources/whatsnew/_index.md

@@ -76,6 +76,7 @@ For a complete list of every change, with links to pull requests and related iss
 
 ## Grafana 10
 
+- [What's new in 10.4](https://grafana.com/docs/grafana/<GRAFANA_VERSION>/whatsnew/whats-new-in-v10-4/)
 - [What's new in 10.3](https://grafana.com/docs/grafana/<GRAFANA_VERSION>/whatsnew/whats-new-in-v10-3/)
 - [What's new in 10.2](https://grafana.com/docs/grafana/<GRAFANA_VERSION>/whatsnew/whats-new-in-v10-2/)
 - [What's new in 10.1]({{< relref "whats-new-in-v10-1/" >}})