build: refine Makefile targets and build command (#666)

* build: refine Makefile targets and build command

- Remove `test` from `.PHONY` and add `install`, `run`, and `clean` targets to `.PHONY`
- Replace build command to include verbose flag and remove `-mod vendor` option

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

* docs: refine README structure and update tests

- Add a "Table of Contents" section to the README
- Reorganize sections within the README, moving "Configuring AWS SES alerts" and "How to change the color thresholds of the response time badge"
- Remove "Sponsors" section from the README
- Update the test command in the README from `go test ./... -mod vendor` to `go test -v ./...`

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

---------

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

Makefile

@@ -1,17 +1,18 @@
 BINARY=gatus
 
-# Because there's a folder called "test", we need to make the target "test" phony
-.PHONY: test
-
+.PHONY: install
 install:
-	go build -mod vendor -o $(BINARY) .
+	go build -v -o $(BINARY) .
 
+.PHONY: run
 run:
 	GATUS_CONFIG_PATH=./config.yaml ./$(BINARY)
 
+.PHONY: clean
 clean:
 	rm $(BINARY)
 
+.PHONY: test
 test:
 	go test ./... -cover