[v10.4.x] Docs: What’s new & Upgrade guide 10.4 (#83944)

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

---------

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>
(cherry picked from commit db13c0839f)

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

docs/sources/upgrade-guide/upgrade-v10.4/index.md

@@ -0,0 +1,37 @@
+---
+description: Guide for upgrading to Grafana v10.4
+keywords:
+  - grafana
+  - configuration
+  - documentation
+  - upgrade
+  - '10.4'
+title: Upgrade to Grafana v10.4
+menuTitle: Upgrade to v10.4
+weight: 1300
+---
+
+# Upgrade to Grafana v10.4
+
+{{< docs/shared lookup="upgrade/intro.md" source="grafana" version="<GRAFANA VERSION>" >}}
+
+{{< docs/shared lookup="back-up/back-up-grafana.md" source="grafana" version="<GRAFANA VERSION>" leveloffset="+1" >}}
+
+{{< docs/shared lookup="upgrade/upgrade-common-tasks.md" source="grafana" version="<GRAFANA VERSION>" >}}
+
+## Technical notes
+
+### Legacy alerting -> Grafana Alerting dry-run on start
+
+If you haven't already upgraded to Grafana Alerting from legacy Alerting, Grafana will initiate a dry-run of the upgrade every time the instance starts. This is in preparation for the removal of legacy Alerting in Grafana v11. The dry-run logs the results of the upgrade attempt and identifies any issues requiring attention before you can successfully execute the upgrade. No changes are made during the dry-run.
+
+You can disable this behavior using the feature flag `alertingUpgradeDryrunOnStart`:
+
+```toml
+[feature_toggles]
+alertingUpgradeDryrunOnStart=false
+```
+
+{{% admonition type="note" %}}
+We strongly encourage you to review the [upgrade guide](https://grafana.com/docs/grafana/v10.4/alerting/set-up/migrating-alerts/) and perform the necessary upgrade steps prior to v11.
+{{% /admonition %}}