remove buck2 from custom variables (#4231)

Summary:
Pull Request resolved: #4231

for alpha+, we need to remove all buck2 commands and buck2 dependencies from static doc and github readmes.
This diff gets rid of the buck2 from custom variable doc.

Reviewed By: kirklandsign

Differential Revision: D59658382

fbshipit-source-id: 11026e30f03afd0fa4e29fccd48f7176c56ddad9

Author: Gasoonjia

docs/README.md

@@ -94,21 +94,18 @@ To build the documentation locally:
 
 You can use custom variables in your `.md` and `.rst` files. The variables take
 their values from the files listed in the `./.ci/docker/ci_commit_pins/`
-directory. For example, to insert a variable that specifies the latest Buck2
+directory. For example, to insert a variable that specifies the latest PyTorch
 version, use the following syntax:
 
 ```
-The current version of Buck2 is ${executorch_version:buck2}.
+The current version of PyTorch is ${executorch_version:pytorch}.
 ```
 
 This will result in the following output:
 
 <img src="./source/_static/img/s_custom_variables_extension.png" width="300">
 
-We have the following custom variables defined in this docset:
-
-- `${executorch_version:buck2}`
-- `${executorch_version:pytorch}`
+Right now we only support PyTorch version as custom variable, but will support others in the future.
 
 You can use the variables in both regular text and code blocks.