Add docs on customising Kanidm. (#3209)

* Add docs on customising Kanidm.

* Add more info about images that can be used.

* s/set-display-name/set-displayname/g

book/src/SUMMARY.md

@@ -24,6 +24,7 @@
   - [Domain Rename](domain_rename.md)
   - [Monitoring the platform](monitoring_the_platform.md)
   - [Recycle Bin](recycle_bin.md)
+  - [Customising](customising.md)
 
 - [Accounts and Groups](accounts/intro.md)
   - [People Accounts](accounts/people_accounts.md)

book/src/customising.md

@@ -0,0 +1,55 @@
+# Customising
+
+> [!NOTE]
+>
+> Currently theming options such as updating the CSS requires modifying the style.css file. This
+> may be changed in the future to make it easier to modify.
+
+Kanidm supports customising various aspects such as the site display name, site image, and display
+names and images for each application.
+
+## Changing the site
+
+### Updating the display Name
+
+By default, the display name is 'Kanidm <hostname>' which is visible when logged in. To modify the
+display name, run the following
+
+```bash
+kanidm system domain set-displayname <new-display-name> -D admin
+```
+
+### Updating the site image
+
+Similarly instead of the default Ferris the crab logo, the image on the signin page can be updated
+or reset with the below commands. The image must satisfy the following conditions:
+1. Maximum 1024 x 1024 pixels
+2. Less than 256 KB
+3. Is a supported image file type: png, jpg, gif, svg, webp
+
+```bash
+kanidm system domain set-image <file-path> [image-type] -D admin
+
+kanidm system domain remove-image -D admin
+```
+
+## Changing a resource server
+
+### Updating the display name
+
+Each application can have its display name updated with the following
+
+```bash
+kanidm system oauth2 set-displayname <NAME> <displayname> -D idm_admin
+```
+
+### Updating the image
+
+Each application can have its image updated or reset with the following commands. The image is
+subject to the same restrictions as the site image above.
+
+```bash
+kanidm system oauth2 set-image <NAME> <file-path> [image-type] -D idm_admin
+
+kanidm system oauth2 remove-image <NAME> -D idm_admin
+```