refactor(templates): move the templates out of the gitea subdirectory

The `templates/gitea` subdirectory did not group anything meaningful, since every template of this chart belongs to
Gitea. It only duplicated the chart name in every path and forced the unit tests to spell out
`templates/gitea/<name>.yaml`, while `_helpers.tpl` and `NOTES.txt` already lived directly in `templates`.

All templates now live in `templates`, which matches the layout of the bundled sub-charts and the Helm defaults.

The checksum helper in `templates/_secrets.tpl` built its include path from `$root.Template.BasePath` and therefore
carried the subdirectory in a `printf` format string instead of a literal path. Without adjusting it the chart failed to
render with "no template gitea/templates/gitea/secret_config.yaml associated with template gotpl".

Co-authored-by: Copilot <copilot@github.com>

README.md

@@ -267,7 +267,7 @@ If `.Values.deployment.gitea.image.rootless: true`, then the following will occu
 
 - `$HOME` becomes `/data/gitea/git`
 
-  [see deployment.yaml](./templates/gitea/deployment.yaml) template inside (init-)container "env" declarations
+  [see deployment.yaml](./templates/deployment.yaml) template inside (init-)container "env" declarations
 
 - `START_SSH_SERVER: true` (Unless explicity overwritten by `gitea.config.server.START_SSH_SERVER`)
 
@@ -279,7 +279,7 @@ If `.Values.deployment.gitea.image.rootless: true`, then the following will occu
 
 - `SSH_LOG_LEVEL` environment variable is not injected into the container
 
-  [see deployment.yaml](./templates/gitea/deployment.yaml) template inside container "env" declarations
+  [see deployment.yaml](./templates/deployment.yaml) template inside container "env" declarations
 
 #### OpenShift Compatibility