Update Extensions doc page

[nstrayer]

Addresses #7557

This PR adds an up to date list of the bundled extensions along with a more detailed explanation of the "built-in" extensions.

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[juliasilge]

I am somewhat hesitant to put a comprehensive list of what we bootstrap install in these docs, because of the maintenance burden involved moving forward. What do you think? Is the clarity worth it? Is having a comprehensive list important to users?

If we didn't have a full list, I certainly would still want to give examples of what we bundle.

[Copilot]

Pull Request Overview

This pull request updates the Extensions documentation page to provide an up-to-date list of bundled extensions and a clearer explanation of built‐in and bootstrapped extensions.

• Revised introductory description to clarify the use of Code OSS and Open VSX.
• Added new sections for “Open VSX”, “Accessing extensions”, and “Included extensions” with separate subsections for built‐in, bootstrapped, and non‐compatible extensions.
• Updated callout notes and links to ensure consistency and clarity.

Comments suppressed due to low confidence (2)

extensions.qmd:7

• [nitpick] Consider adding a brief explanation for the vale Posit.Contractions annotations to assist less technical readers in understanding their purpose.

<!-- vale Posit.Contractions = NO -->

extensions.qmd:72

• Confirm that the updated link for reporting issues with the Python extension is accurate and consistent with the other documentation references.

-   [Python extension](https://marketplace.visualstudio.com/items?itemName=ms-python.python): Positron bundles a fork of this extension that's built to work with Positron and offers support for the console, help, and other features. If there is anything from the original Python extension that doesn't work for you, please [let us know](https://github.com/posit-dev/positron/issues).

[juliasilge]

Thank you so much! 🙌 It is really great to get this updated information ready to go.