Update swagger API specification (#1782)

# Summary

This PR drops the outdated former swagger.yaml/json and introduced
automatic API document generation from Go code.
The generated code is also used to generate documentation/markdown for
the community page,
as well as enable the Woodpecker server to serve a Swagger Web UI for
manual tinkering.

I did opt-in for gin-swagger, a middleware for the Gin framework, to
ease implementation and have a sophisticated output.
This middleware only produces Swagger v2 specs. AFAIK the newer OpenApi
3x tooling is not yet that mature,
so I guess that's fine for now.

## Implemenation notes

- former swagger.json files removed
- former // swagger godocs removed
- introduced new dependency gin-swagger, which uses godoc annotations on
top of Gin Handler functions.
- reworked Makefile to automatically generate Go code for the server
- introduce new dependency go-swagger, to generate Markdown for
documentation purposes
- add a Swagger Web UI, incl. capabilities for manual API exploration
- consider relative root paths in the implementation
- write documentation for all exposed API endpoints
- incl. API docs in the community website (auto-generated)
- provide developer documentation, for the Woodpecker authors
- no other existing logic/code was intentionally changed

---------

close #292

---------

Co-authored-by: qwerty287 <80460567+qwerty287@users.noreply.github.com>
Co-authored-by: 6543 <6543@obermui.de>

cmd/server/swagger.go

@@ -0,0 +1,38 @@
+// Copyright 2023 Woodpecker Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import (
+	"github.com/woodpecker-ci/woodpecker/cmd/server/docs"
+	"github.com/woodpecker-ci/woodpecker/version"
+)
+
+// generate docs/docs/20-usage/90-rest-api.md via:
+//go:generate go run woodpecker_docs_gen.go swagger.go
+
+// setupSwaggerStaticConfig initializes static content only (contacts, title and description)
+// for dynamic configuration of e.g. hostname, etc. see router.setupSwaggerConfigAndRoutes
+//
+//	@contact.name	Woodpecker CI Community
+//	@contact.url	https://woodpecker-ci.org/
+func setupSwaggerStaticConfig() {
+	docs.SwaggerInfo.BasePath = "/api"
+	docs.SwaggerInfo.InfoInstanceName = "api"
+	docs.SwaggerInfo.Title = "Woodpecker CI API"
+	docs.SwaggerInfo.Version = version.String()
+	docs.SwaggerInfo.Description = "Woodpecker is a simple CI engine with great extensibility.\n" +
+		"To get a personal access token (PAT) for authentication, please log in your Woodpecker server,\n" +
+		"and go to you personal profile page, by clicking the user icon at the top right."
+}