Set a doc build variable for executorch version

[kirklandsign]

Summary

Now in docs we can use ${executorch_version} to represent executorch version.

It's defined in docs/source/executorch_custom_versions.py EXECUTORCH_VERSION = "0.7.0"

Test plan

In https://docs-preview.pytorch.org/pytorch/executorch/13351/getting-started.html#id1, the version is correct:

implementation("org.pytorch:executorch-android:0.7.0")

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/13351

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 3 New Failures, 1 Unrelated Failure

As of commit 4c03d30 with merge base 76a4062 ():

NEW FAILURES - The following jobs have failed:

• pull / test-vulkan-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t b361296d9759e5edc3ab8a3edfff7fa929d6628479ce4c29af3def7aba670cc6 /exec failed with exit code 139
• pull / unittest-arm-backend-with-no-fvp (test_pytest_ops) / linux-job (gh)
  RuntimeError: Command docker exec -t 6f55a20d5f17bd1c8c2817870dc2db81f0e141d391bd4333a478108da90302c5 /exec failed with exit code 1
• pull / unittest-buck / macos / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 3

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest-arm-backend-with-no-fvp (test_pytest_models) / linux-job (gh) (trunk failure)
  backends/arm/test/models/stable_diffusion/test_vae_AutoencoderKL.py::TestAutoencoderKL::test_AutoencoderKL_tosa_MI

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