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This PR drops the outdated former swagger.yaml/json and introduced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/main.go

@@ -20,6 +20,7 @@ import (
 
 	_ "github.com/joho/godotenv/autoload"
 	"github.com/urfave/cli/v2"
+	_ "github.com/woodpecker-ci/woodpecker/cmd/server/docs"
 
 	"github.com/woodpecker-ci/woodpecker/version"
 )
@@ -32,6 +33,8 @@ func main() {
 	app.Action = run
 	app.Flags = flags
 
+	setupSwaggerStaticConfig()
+
 	if err := app.Run(os.Args); err != nil {
 		_, _ = fmt.Fprintln(os.Stderr, err)
 		os.Exit(1)