fix: use H1 for page title in backup, remove and upgrade docs (#621)

* Change header from H2 to H1 in backup.mdx

With H2 header, the browser tab name was "undefined" instead of the proper name

* Change H2 header to H1 in remove.mdx

With H2 header, the browser tab name was "undefined" instead of the proper name

* Change H2 header to H1 in upgrade.mdx

With H2 header, the browser tab name was "undefined" instead of the proper name

* Promote h3 sub-headings to h2 in upgrade.mdx to fix TOC error

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Jack Carter <128555021+SunsetDrifter@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: 3 people

src/pages/selfhosted/maintenance/backup.mdx

@@ -1,4 +1,4 @@
-## Back Up Your Self-Hosted NetBird Installation
+# Back Up Your Self-Hosted NetBird Installation
 
 To back up your NetBird installation, you need to copy the configuration files and the Management service databases.
 

src/pages/selfhosted/maintenance/remove.mdx

@@ -1,4 +1,4 @@
-## Remove Your Self-Hosted NetBird Installation
+# Remove Your Self-Hosted NetBird Installation
 
 To remove the NetBird installation and all related data from your server, run these commands from the folder where you installed NetBird:
 ```bash

src/pages/selfhosted/maintenance/upgrade.mdx

@@ -1,6 +1,6 @@
-## Upgrade Your Self-Hosted NetBird Installation
+# Upgrade Your Self-Hosted NetBird Installation
 
-### Check for Updates
+## Check for Updates
 
 The NetBird Dashboard displays an update indicator at the bottom of the left navigation menu when a new version is available:
 
@@ -27,7 +27,7 @@ Example response:
 }
 ```
 
-### Check Release Notes
+## Check Release Notes
 
 Before upgrading, review the release notes for any breaking changes or migration steps:
 
@@ -38,7 +38,7 @@ Before upgrading, review the release notes for any breaking changes or migration
 Management, Signal, and Relay are all part of the same repository and share the same version numbers.
 </Note>
 
-### Upgrade Steps
+## Upgrade Steps
 
 To upgrade NetBird to the latest version:
 
@@ -57,7 +57,7 @@ To upgrade NetBird to the latest version:
 For upgrades from older versions (pre-v0.26.0), see the [Legacy upgrade notes](#legacy-self-hosting-with-zitadel-idp) below.
 </Note>
 
-### Legacy Setup (Separate Containers)
+## Legacy Setup (Separate Containers)
 
 If your deployment uses the older setup with separate containers (`management`, `signal`, `relay`, `coturn`), pull and recreate those containers instead: