Telemetry: track Sphinx extensions and html_theme variables (#9639)

* Telemetry: track Sphinx `extensions` and `html_theme` variables

Pretty simple implementation to track Sphinx's extensions and theme.
This is not the best implementation, but I think it could be good as a first
start.

The new `doctool` key added will have this shape:

```
  "doctool": {
    "extensions": [
      "readthedocs_ext.readthedocs",
      "notfound.extension",
      "autoapi.extension",
      "sphinx_tabs.tabs",
      "sphinx-prompt",
      "sphinxemoji.sphinxemoji"
    ],
    "name": "sphinx",
    "theme": "sphinx_rtd_theme"
  },
```

Closes #9627

* Lint

* Lint

* Use `Version.is___type()` to know if it's Sphinx/MkDocs or generic

* Telemetry: use `readthedocs-sphinx-ext`'s dump to load the data

Our extension is now dumping the data we want to consume here. So, we are just
reading that file and saving it as part of the `BuildData` object.

* These are properties, not functions

* Missing `return`

* Return `doctool` data even if the file does not exist

readthedocs/telemetry/collectors.py

@@ -1,6 +1,7 @@
 """Data collectors."""
 
 import json
+import os
 
 import dparse
 import structlog
@@ -89,6 +90,37 @@ def collect(self):
                 "all": all_apt_packages,
             },
         }
+        data["doctool"] = self._get_doctool()
+        return data
+
+    def _get_doctool_name(self):
+        if self.version.is_sphinx_type:
+            return "sphinx"
+
+        if self.version.is_mkdocs_type:
+            return "mkdocs"
+
+        return "generic"
+
+    def _get_doctool(self):
+        data = {
+            "name": self._get_doctool_name(),
+            "extensions": [],
+            "html_theme": "",
+        }
+
+        if self._get_doctool_name() != "sphinx":
+            return data
+
+        conf_py_dir = os.path.join(
+            self.checkout_path,
+            os.path.dirname(self.config.sphinx.configuration),
+        )
+        filepath = os.path.join(conf_py_dir, "_build", "json", "telemetry.json")
+        if os.path.exists(filepath):
+            with open(filepath, "r") as json_file:
+                content = json_file.read()
+            data.update(self._safe_json_loads(content, {}))
         return data
 
     def _get_all_conda_packages(self):