feat: add Gateway API support (#1073)

Add full Gateway API support for exposing Gitea via HTTPRoute, TCPRoute, BackendTLSPolicy, and ClientSettingsPolicy resources.

New templates:
- `httpRoute.yaml` — renders an HTTPRoute with configurable
  parentRefs, hostnames, and rules (defaults to PathPrefix `/`)
- `tcpRoute.yaml` — renders a TCPRoute for SSH traffic
- `backendTLSPolicy.yaml` — renders a BackendTLSPolicy for
  encrypted backend connections with required validation config
- `clientSettingsPolicy.yaml` — renders an NGINX Gateway Fabric
  ClientSettingsPolicy to raise the request body size limit

Infrastructure:
- `gatewayAPI.enabled` global toggle gates all resources
- Resources grouped under `gatewayAPI.core.*` and `gatewayAPI.nginx.*`
- Helper templates extracted into dedicated `_*.tpl` files
- Service name helpers (`gitea.service.http.name`, `gitea.service.ssh.name`)
  extracted into `_services.tpl`; service templates renamed to camelCase
- `ROOT_URL`, `DOMAIN`, and `SSH_DOMAIN` auto-resolve from
  `httpRoute.hostnames[0]`; `httpRoute.tls` switches to `https`

Documentation:
- New `docs/gateway-api.md` with topology examples, BackendTLSPolicy
  setup, sectionName guidance, SSH considerations, and NGINX body
  size limit configuration
- `.github/copilot-instructions.md` with project conventions
- README parameter table auto-generated via `make readme`

Tests:
- Helm unit tests for all four new resource templates
- Config tests for hostname/TLS resolution from Gateway API values

Co-authored-by: Todd Marimon <toddmarimon@gmail.com>

templates/gitea/_backendTLSPolicies.tpl

@@ -0,0 +1,30 @@
+{{/* vim: set filetype=mustache: */}}
+
+{{/* annotations */}}
+
+{{- define "gitea.backendTLSPolicy.annotations" -}}
+{{- with .Values.gatewayAPI.core.backendTLSPolicy.annotations }}
+{{- toYaml . -}}
+{{- end }}
+{{- end }}
+
+{{/* enabled */}}
+
+{{- define "gitea.backendTLSPolicy.enabled" -}}
+{{- if and .Values.gatewayAPI.enabled
+          .Values.gatewayAPI.core.backendTLSPolicy.enabled
+-}}
+true
+{{- else -}}
+false
+{{- end -}}
+{{- end }}
+
+{{/* labels */}}
+
+{{- define "gitea.backendTLSPolicy.labels" -}}
+{{ include "gitea.labels" . }}
+{{- with .Values.gatewayAPI.core.backendTLSPolicy.labels }}
+{{ toYaml . }}
+{{- end }}
+{{- end }}