Update doc to point to 0.7 as stable (#13321)

Summary:

Ran this command `ln -sfn 0.7 stable`

### Summary
[PLEASE REMOVE] See [CONTRIBUTING.md's Pull
Requests](https://github.com/pytorch/executorch/blob/main/CONTRIBUTING.md#pull-requests)
for ExecuTorch PR guidelines.

[PLEASE REMOVE] If this PR closes an issue, please add a `Fixes
#<issue-id>` line.

[PLEASE REMOVE] If this PR introduces a fix or feature that should be
the upcoming release notes, please add a "Release notes: <area>" label.
For a list of available release notes labels, check out
[CONTRIBUTING.md's Pull
Requests](https://github.com/pytorch/executorch/blob/main/CONTRIBUTING.md#pull-requests).

### Test plan
[PLEASE REMOVE] How did you test this PR? Please write down any manual
commands you used and note down tests that you have written if
applicable.

Author: mergennachin

stable

@@ -1 +1 @@
-0.6
+0.7

versions.html

@@ -20,10 +20,10 @@ <h1> ExecuTorch Documentation </h1>
           <a class="reference internal" href="main/">main (unstable)</a>
         </li>
         <li class="toctree-l1">
-          <a class="reference internal" href="0.7">0.7 (release candidate)</a>
+          <a class="reference internal" href="0.7">0.7 (stable release)</a>
         </li>
         <li class="toctree-l1">
-          <a class="reference internal" href="0.6">0.6 (stable release)</a>
+          <a class="reference internal" href="0.6">0.6 </a>
         </li>
         <li class="toctree-l1">
           <a class="reference internal" href="0.5">0.5</a>