Onboard onto bakery: bakery.toml + dev/beta/release CI, relocate content paths

Adds bakery.toml (binaries, license_file, desktop_file, data_archive for
the guide content directory) and the standard three-track CI workflows,
mirroring the pattern used across the rest of the bread ecosystem.

Also fixes the tour/troubleshooting-symptoms loaders (tour.rs,
troubleshoot.rs) to check the bakery-writable user content root
(~/.local/share/breadhelp/content) before the system path a pacman
package would have used — the main ContentStore already did this via
user_content_root(), these two were the last holdouts still hardcoded to
/usr/share/breadhelp only.

bakery.toml

@@ -0,0 +1,12 @@
+name = "breadhelp"
+description = "Onboarding and help center for Bread OS"
+binaries = ["breadhelp"]
+system_deps = ["gtk4", "glib2", "gtk4-layer-shell", "hicolor-icon-theme"]
+optional_system_deps = ["snapper"]
+bread_deps = []
+license_file = "LICENSE"
+desktop_file = "breadhelp.desktop"
+data_archive = "content.tar.gz"
+
+[install]
+post_install = []