Build: don't run default steps if no sphinx or mkdocs

This PR modifies the build to not run default commands when the user doesn't
specify `sphinx` or `mkdocs` configs in the YAML file.

This allows to use the following YAML file to build with Docusaurus, for
example:

```yaml
version: 2

build:
  os: ubuntu-24.04
  tools:
    nodejs: "22"
  jobs:
    install:
      - cd docs/ && npm install
    build:
      html:
        - cd docs/ && npm run build
    post_build:
      - mkdir --parents $READTHEDOCS_OUTPUT/html/
      - cp --recursive docs/build/* $READTHEDOCS_OUTPUT/html/
```

This is the structure I've been wanting to have to allow any type of doctool to
work in our platform following the `build.jobs` pattern.

The code deployed in production doesn't allow this because it runs automatically
`create_environment`, `install_core_dependencies` and `install_requirements`
even if the project is not using Python --which is incorrect. It fails with the
generic error: " Unknown problem. There was a problem with Read the Docs while
building your documentation"

- https://app.readthedocs.org/projects/test-builds/builds/26425680/
- https://read-the-docs.sentry.io/issues/4458662274/

This PR detects if the user is defining `sphinx` or `mkdocs` and if it's not, it
doesn't run those jobs and leave the user to handle the build workflow
completely.

* Related #11216
* Related #11551

readthedocs/config/config.py

@@ -828,7 +828,14 @@ def doctype(self):
 
         if self.mkdocs:
             return "mkdocs"
-        return self.sphinx.builder
+
+        # NOTE: we need to use "source config" here because `_config` is
+        # auto-populated with Sphinx if no `sphinx` and no `mkdocs` keys are
+        # declared.
+        if self.source_config.get("sphinx"):
+            return self.sphinx.builder
+
+        return GENERIC
 
     @property
     def submodules(self):

readthedocs/doc_builder/director.py

@@ -107,6 +107,8 @@ def setup_vcs(self):
         # self.run_build_job("pre_checkout")
         self.checkout()
 
+        self.run_default_commands = self.is_type_mkdocs() or self.is_type_sphinx()
+
         self.run_build_job("post_checkout")
 
         commit = self.data.build_commit or self.vcs_repository.commit
@@ -309,23 +311,28 @@ def create_environment(self):
         if self.data.config.build.jobs.create_environment is not None:
             self.run_build_job("create_environment")
             return
-        self.language_environment.setup_base()
+
+        if self.run_default_commands or self.data.config.source_config.get("python"):
+            self.language_environment.setup_base()
 
     # Install
     def install(self):
         if self.data.config.build.jobs.install is not None:
             self.run_build_job("install")
             return
 
-        self.language_environment.install_core_requirements()
-        self.language_environment.install_requirements()
+        if self.run_default_commands or self.data.config.source_config.get("python"):
+            self.language_environment.install_core_requirements()
+            self.language_environment.install_requirements()
 
     # Build
     def build_html(self):
         if self.data.config.build.jobs.build.html is not None:
             self.run_build_job("build.html")
             return
-        return self.build_docs_class(self.data.config.doctype)
+
+        if self.run_default_commands:
+            return self.build_docs_class(self.data.config.doctype)
 
     def build_pdf(self):
         if "pdf" not in self.data.config.formats or self.data.version.type == EXTERNAL:
@@ -336,7 +343,7 @@ def build_pdf(self):
             return
 
         # Mkdocs has no pdf generation currently.
-        if self.is_type_sphinx():
+        if self.is_type_sphinx() and self.run_default_commands:
             return self.build_docs_class("sphinx_pdf")
 
         return False
@@ -353,7 +360,7 @@ def build_htmlzip(self):
             return
 
         # We don't generate a zip for mkdocs currently.
-        if self.is_type_sphinx():
+        if self.is_type_sphinx() and self.run_default_commands:
             return self.build_docs_class("sphinx_singlehtmllocalmedia")
         return False
 
@@ -366,7 +373,7 @@ def build_epub(self):
             return
 
         # Mkdocs has no epub generation currently.
-        if self.is_type_sphinx():
+        if self.is_type_sphinx() and self.run_default_commands:
             return self.build_docs_class("sphinx_epub")
         return False
 
@@ -744,6 +751,10 @@ def is_type_sphinx(self):
         """Is documentation type Sphinx."""
         return "sphinx" in self.data.config.doctype
 
+    def is_type_mkdocs(self):
+        """Is documentation type MkDocs."""
+        return "mkdocs" in self.data.config.doctype
+
     def store_readthedocs_build_yaml(self):
         # load YAML from user
         yaml_path = os.path.join(