Update the "This is an alpha release" wording to "this is beta release" (#6217)

Summary: Pull Request resolved: #6217

Reviewed By: tarun292

Differential Revision: D64374464

fbshipit-source-id: 8f468d09446cab387bdefc9e19605ef7fed544de
(cherry picked from commit 59070e5)

Author: mergennachin

docs/source/index.rst

@@ -4,11 +4,12 @@ Welcome to the ExecuTorch Documentation
 =======================================
 
 .. important::
-   This is an alpha release; the ExecuTorch APIs and the ``.pte`` binary format
-   may change in incompatible ways before stabilizing in a future beta release.
-   When deploying models, we currently recommend using a version of the runtime
-   built from the same git revision that was used to generate the ``.pte`` file.
-   Once the format has stabilized, this will no longer be necessary.
+   This is a beta release. As of this ExecuTorch beta release, the API
+   will follow the `lifecycle and deprecation policy <api-life-cycle.html.html>`__
+   and ``.pte`` binary format will comply with the *runtime compatibility policy* (TODO: add link).
+   This ensures that application developers can update to the latest version of ExecuTorch
+   without breaking existing integration code, in accordance with these policies.
+   If any issues arise or compatibility breaks occur, please `report them in GitHub <https://github.com/pytorch/executorch/issues/new/choose>`__.
 
    We welcome any feedback, suggestions, and bug reports from the community
    to help us improve the technology. Please use the `PyTorch Forums