doc: add clone command to technical reference §9

Put the Gitea clone command at the start of Build & deploy, since
getting the source is the first build step. (Kept out of the user
guide, which is end-user-only.)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

doc/Chatter_technical_reference.md

@@ -288,6 +288,14 @@ its full refreshes too); we minimized *when* it happens rather than eliminating
 
 ## 9. Build & deploy
 
+**Get the source:** clone from the Gitea server (needs an account with read
+access to `ack/chatter` and your SSH key registered, or use the HTTPS URL):
+
+```
+git clone git@git.andykopra.com:ack/chatter.git
+cd chatter
+```
+
 **Toolchain:** official reMarkable Chiappa SDK `3.27.0.97` (Qt 6.8.2 sysroot,
 x86_64 host). Install, then `source environment-setup-cortexa55-remarkable-linux`.