doc(securing-your-webhooks): add standard JavaScript example

[coolaj86]

Adds a JavaScript example that is secure, and follows the WebCrypto standards to work in all JavaScript environments.

Why:

To add a cross-runtime JavaScript example

What's being changed (if available, include any code snippets, screenshots, or gifs):

Adds a cross-runtime JavaScript example

Check off the following:

• I have reviewed my changes in staging, available via the View deployment link in this PR's timeline.

  • For content changes, you will also see an automatically generated comment with links directly to pages you've modified. The comment won't appear if your PR only edits files in the data directory.

• For content changes, I have completed the self-review checklist.

[github-actions]

Automatically generated comment ℹ️

This comment is automatically generated and will be overwritten every time changes are committed to this branch.

The table contains an overview of files in the content directory that have been changed in this pull request. It's provided to make it easy to review your changes on the staging site. Please note that changes to the data directory will not show up in this table.

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Content directory changes

You may find it useful to copy this table into the pull request summary. There you can edit it to share links to important articles or changes and to give a high-level overview of how the changes in your pull request support the overall goals of the pull request.

Source	Preview	Production	What Changed
webhooks-and-events/webhooks/securing-your-webhooks.md	fpt ghec ghes@ 3.10 3.9 3.8 3.7 3.6 ghae	fpt ghec ghes@ 3.10 3.9 3.8 3.7 3.6 ghae

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fpt: Free, Pro, Team
ghec: GitHub Enterprise Cloud
ghes: GitHub Enterprise Server
ghae: GitHub AE

[cmwilson21]

@coolaj86 Thanks so much for submitting a PR! I'll get this triaged for review ⚡

[skedwards88]

Thanks for adding this! I'm going to incorporate one minor suggestion, then we'll get this merged for you.

[skedwards88]

Thanks for adding this! I'm going to incorporate one minor suggestion, then we'll get this merged for you.

[peterbe]

@skedwards88 @cmwilson21
The Azure preview environment deployment will start to work again once https://github.com/github/docs-internal/pull/40540 has landed in docs-internal and later been repo-sync'ed to this repo.

[github-actions]

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