feat(*): support serving API documentation pages when the server starts

[iawia002]

What this PR does / why we need it:

Support serving API documentation pages when the server starts, take examples/api-basic project as an example:

INFO  1023-18:11:50.982+08 config.go:309 | Listening on :8080
INFO  1023-18:11:52.009+08 builder.go:240 | Definitions: 1 Middlewares: 0 Path: /docs/api.v2.json
INFO  1023-18:11:52.009+08 builder.go:255 |   Method: Get Consumes: [] Produces: [application/json]
INFO  1023-18:11:52.009+08 builder.go:240 | Definitions: 1 Middlewares: 0 Path: /docs/api.v1.json
INFO  1023-18:11:52.009+08 builder.go:255 |   Method: Get Consumes: [] Produces: [application/json]
INFO  1023-18:11:52.009+08 builder.go:240 | Definitions: 1 Middlewares: 0 Path: /docs
INFO  1023-18:11:52.009+08 builder.go:255 |   Method: Get Consumes: [] Produces: [text/html]
INFO  1023-18:11:52.009+08 builder.go:240 | Definitions: 1 Middlewares: 0 Path: /api/v1/applications
INFO  1023-18:11:52.009+08 builder.go:255 |   Method: Create Consumes: [application/json] Produces: [application/json]
INFO  1023-18:11:52.009+08 builder.go:240 | Definitions: 1 Middlewares: 0 Path: /api/v2/applications
INFO  1023-18:11:52.009+08 builder.go:255 |   Method: Create Consumes: [application/json] Produces: [application/json]

Which issue(s) this PR is related to (optional, link to 3rd issue(s)):

Fixes #

Reference to #302

Special notes for your reviewer:

/cc @xpofei

/kind feature

Release note:

Support serving API documentation pages when the server starts.

[xpofei]

/lgtm

[iawia002]

/approve

[caicloud-bot]

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