[doc] add a doc page about ci testing workflow on a PR (#45446)

As title, add a doc page about the new ci testing workflow (microcheck,
go, premerge, etc.). I'll follow up with another PR to add more images
to the doc.

Test:
-
https://anyscale-ray--45446.com.readthedocs.build/en/45446/ray-contribute/ci.html

<img width="1622" alt="Screenshot 2024-05-20 at 11 02 29 AM"
src="https://github.com/ray-project/ray/assets/128072568/46a4c056-d9fd-47ef-ae28-40001bbabc41">

---------

Signed-off-by: can <[email protected]>
Signed-off-by: Cuong Nguyen <[email protected]>
Co-authored-by: angelinalg <[email protected]>

doc/source/ray-contribute/ci.rst

@@ -0,0 +1,52 @@
+CI Testing Workflow on PRs
+==========================
+
+This guide helps contributors to understand the Continuous Integration (CI)
+workflow on a PR. Here CI stands for the automated testing of the codebase
+on the PR.
+
+`microcheck`: default tests on your PR
+--------------------------------------
+With every commit on your PR, by default, we'll run a set of tests
+called `microcheck`.
+
+These tests are designed to be 90% accurate at catching bugs on your
+PR while running only 10% of the full test suite. As a result,
+microcheck typically finishes twice as fast and twice cheaper than
+the full test suite. Some of the notable features of microcheck are:
+
+* If a new test is added or an existing test is modified in a pull
+  request, microcheck will ensure these tests are included.
+* You can manually add more tests to microcheck by including the following line
+  in the body of your git commit message:
+  `@microcheck TEST_TARGET01 TEST_TARGET02 ....`. This line must be in the
+  body of your message, starting from the second line or
+  below (the first line is the commit message title). For example, here
+  is how I manually add tests in my pull request::
+
+    // git command to add commit message
+    git commit -a -s
+
+    // content of the commit message
+    run other serve doc tests
+
+    @microcheck //doc:source/serve/doc_code/distilbert //doc:source/serve/doc_code/object_detection //doc:source/serve/doc_code/stable_diffusion
+
+    Signed-off-by: can <[email protected]>
+
+If microcheck passes, you'll see a green checkmark on your PR. If it
+fails, you'll see a red cross. In either case, you'll see a summary of
+the test run statuses in the github UI.
+
+
+Additional tests at merge time
+------------------------------
+In this workflow, to merge your PR, simply click on the Enable auto-merge
+button (or ask a committer to do so). This will trigger additional test
+cases, and the PR will merge automatically once they finish and pass.
+
+Alternatively, you can also add a `go` label to manually trigger the full
+test suite on your PR (be mindful that this is less recommended but we
+understand you know best about the need of your PR). While we anticipate
+this will be rarely needed, if you do require it constantly, please let
+us know. We are continuously improve the effectiveness of microcheck.

doc/source/ray-contribute/getting-involved.rst

@@ -10,6 +10,7 @@ Getting Involved / Contributing
     :hidden:
 
     development
+    ci
     docs
     writing-code-snippets
     fake-autoscaler