Add Braintree VBE and update UG links

[leslietilling]

Purpose of this pull request

This pull request (PR) updates the VBE page to include Braintree as a VBE and update the URLs for user guide links using the old URL path.

Affected DevDocs pages

• https://devdocs.magento.com/extensions/vendor/

[jeff-matthews]

Do we need to add Braintree to the compatibility matrix?

[leslietilling]

Do we need to add Braintree to the compatibility matrix?

Yes, we should. In 2.4.0, it was removed as core integration and added as VBE.

[dobooth]

@leslietilling

[leslietilling]

@jeff-matthews This is the version information from Daisy:

Braintree Version| Magento Version
v4.1.0 | v2.4.0
v4.2.0| v2.4.1

[jeff-matthews]

I'll add the 2.4.0 entry in this PR and open a new PR to the 2.4.2-develop branch to add the 2.4.1 entry.

[jeff-matthews]

running tests

[ghost]

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[jeff-matthews]

Oops... I just realized that we didn't update the matrix for 2.4.1 and I confused myself thinking that 2.4.1 was the next release.

I've added 2.4.1 entries for all other VBEs in the 2.4.2-develop branch, so I'm going to stick to the plan and add the Braintree entry for 2.4.1 there as well. We've gone this long, what's a few more days?