Document all API endpoints

[nitrocode]

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• I'd be willing to implement this feature (contributing guide)

Describe the user story
There are many api endpoints and users are unaware. There is no document on the website so new PRs for additional endpoints do not feel the need to document.

Ref #997

Describe the solution you'd like

Create an initial document to start off the docs or document all the api endpoints here

atlantis/server/server.go

Lines 845 to 863 in 3bfd769

	func (s *Server) Start() error {
	s.Router.HandleFunc("/", s.Index).Methods("GET").MatcherFunc(func(r *http.Request, rm *mux.RouteMatch) bool {
	return r.URL.Path == "/" || r.URL.Path == "/index.html"
	})
	s.Router.HandleFunc("/healthz", s.Healthz).Methods("GET")
	s.Router.HandleFunc("/status", s.StatusController.Get).Methods("GET")
	s.Router.PathPrefix("/static/").Handler(http.FileServer(&assetfs.AssetFS{Asset: static.Asset, AssetDir: static.AssetDir, AssetInfo: static.AssetInfo}))
	s.Router.HandleFunc("/events", s.VCSEventsController.Post).Methods("POST")
	s.Router.HandleFunc("/api/plan", s.APIController.Plan).Methods("POST")
	s.Router.HandleFunc("/api/apply", s.APIController.Apply).Methods("POST")
	s.Router.HandleFunc("/github-app/exchange-code", s.GithubAppController.ExchangeCode).Methods("GET")
	s.Router.HandleFunc("/github-app/setup", s.GithubAppController.New).Methods("GET")
	s.Router.HandleFunc("/apply/lock", s.LocksController.LockApply).Methods("POST").Queries()
	s.Router.HandleFunc("/apply/unlock", s.LocksController.UnlockApply).Methods("DELETE").Queries()
	s.Router.HandleFunc("/locks", s.LocksController.DeleteLock).Methods("DELETE").Queries("id", "{id:.*}")
	s.Router.HandleFunc("/lock", s.LocksController.GetLock).Methods("GET").
	Queries(LockViewRouteIDQueryParam, fmt.Sprintf("{%s}", LockViewRouteIDQueryParam)).Name(LockViewRouteName)
	s.Router.HandleFunc("/jobs/{job-id}", s.JobsController.GetProjectJobs).Methods("GET").Name(ProjectJobsViewRouteName)
	s.Router.HandleFunc("/jobs/{job-id}/ws", s.JobsController.GetProjectJobsWS).Methods("GET")

In the future, we can look into swagger documentation using a library that works well with gorilla mux.

https://onfolk.com/blog/swagger-with-go-walkthrough/

Describe the drawbacks of your solution

Need time to spend

Describe alternatives you've considered

Ignoring this problem