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This PR drops the outdated former swagger.yaml/json and introduced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

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close #292

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

server/model/secret.go

@@ -68,7 +68,6 @@ type SecretStore interface {
 }
 
 // Secret represents a secret variable, such as a password or token.
-// swagger:model registry
 type Secret struct {
 	ID          int64          `json:"id"              xorm:"pk autoincr 'secret_id'"`
 	Owner       string         `json:"-"               xorm:"NOT NULL DEFAULT '' UNIQUE(s) INDEX 'secret_owner'"`
@@ -80,7 +79,7 @@ type Secret struct {
 	Events      []WebhookEvent `json:"event"           xorm:"json 'secret_events'"`
 	SkipVerify  bool           `json:"-"               xorm:"secret_skip_verify"`
 	Conceal     bool           `json:"-"               xorm:"secret_conceal"`
-}
+} //	@name Secret
 
 // TableName return database table name for xorm
 func (Secret) TableName() string {