github app: generate jwt with powershell

[mac2000]

Why:

PowerShell is quite powerful and adopted alternative to Bash and Python, even more, it is available out of the box in GitHub Actions.

Will be so nice to have an example of how we can sign JWT token in it, so anyone will not spent time trying to figure this out in future

The good news - everything is done without the need of any 3rd party dependencies so will work everywhere

While trying to figure out how to do it, instead of making notes to my notebook decided why not to ask for such change here, right in docs

Closes: #30678

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

No changes, addition of one more example

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Content directory changes

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Source	Preview	Production	What Changed
apps/creating-github-apps/authenticating-with-a-github-app/generating-a-json-web-token-jwt-for-a-github-app.md	fpt ghec ghes@ 3.11 3.10 3.9 3.8 3.7 ghae	fpt ghec ghes@ 3.11 3.10 3.9 3.8 3.7 ghae

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

[nguyenalex836]

@mac2000 Thanks so much for opening a PR! I'll get this triaged for review ✨

[mac2000]

Hm, something strange Content Lint Markdown / lint-content does not like neither pwsh nor ps1 🤷‍♂️

[skedwards88]

Thanks for adding this example! I made some suggestions to use placeholders in accordance with the style guide; I'll incorporate those suggestions and then get this merged for you.

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