Build: add support for Ruby (#11075)

Closes #10346
Requires readthedocs/readthedocs-docker-images#200

Author: humitos

docs/user/config-file/v2.rst

@@ -283,7 +283,7 @@ build.tools
 Version specifiers for each tool. It must contain at least one tool.
 
 :Type: ``dict``
-:Options: ``python``, ``nodejs``, ``rust``, ``golang``
+:Options: ``python``, ``nodejs``, ``ruby``, ``rust``, ``golang``
 :Required: ``true``
 
 build.tools.python
@@ -326,6 +326,15 @@ Node.js version to use.
    - ``19``
    - ``20``
 
+build.tools.ruby
+````````````````
+
+Ruby version to use.
+
+:Type: ``string``
+:Options:
+   - ``3.3``
+
 build.tools.rust
 ````````````````
 

readthedocs/builds/constants_docker.py

@@ -50,6 +50,9 @@
             "19": "19.0.1",
             "20": "20.11.0",  # LTS
         },
+        "ruby": {
+            "3.3": "3.3.0",
+        },
         "rust": {
             "1.55": "1.55.0",
             "1.61": "1.61.0",