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/_helpers.tpl

@@ -247,6 +247,8 @@ app.kubernetes.io/instance: {{ .Release.Name }}
 {{- define "gitea.public_hostname" -}}
 {{- if and .Values.route.enabled .Values.route.host -}}
 {{ tpl .Values.route.host . }}
+{{- else if and .Values.gatewayAPI.enabled .Values.gatewayAPI.core.httpRoute.enabled (gt (len .Values.gatewayAPI.core.httpRoute.hostnames) 0) -}}
+{{ tpl (index .Values.gatewayAPI.core.httpRoute.hostnames 0) $ }}
 {{- else if gt (len .Values.ingress.hosts) 0 -}}
 {{ tpl (index .Values.ingress.hosts 0).host $ }}
 {{- else -}}
@@ -308,6 +310,8 @@ app.kubernetes.io/instance: {{ .Release.Name }}
 https
 {{- else if and .Values.ingress.enabled (gt (len .Values.ingress.tls) 0) -}}
 https
+{{- else if and .Values.gatewayAPI.enabled .Values.gatewayAPI.core.httpRoute.enabled .Values.gatewayAPI.core.httpRoute.tls -}}
+https
 {{- else -}}
 {{ .Values.gitea.config.server.PROTOCOL }}
 {{- end -}}