Azure Key Vault application variable provider support in Spin 2.5

[ThorstenHans]

This PR adds documentation for using Azure Key Vault as application variable provider.

This should not be merged before the corresponding sample has been merged (https://github.com/fermyon/spin/pull/2483)

Pre-Merge Content Checklist

This documentation has been checked to ensure that:

• [x] The title, template, and date are all set
• [x] Does this PR have a new menu item (anywhere in templates/*.hbs files) that points to a document .md that is set to publish in the future? If so please only publish the .md and .hbs changes in real-time (otherwise there will be a menu item pointing to a .md file that does not exist)
• [x] File does not use CRLF, but uses plain LF (hint: use cat -ve <filename> | grep '^M' | wc -l and expect 0 as a result)
• [x] Has passed bart check
• [x] Has been manually tested by running in Spin/Bartholomew (hint: use PREVIEW_MODE=1 and run npm run styles to update styling)
• [x] Headings are using Title Case
• [x] Code blocks have the programming language set to properly highlight syntax and the proper copy directive
• [x] Have tested with npm run test and resolved all errors
• [x] Relates to an existing (potentially outdated) blog article? If so please add URL in blog to point to this content.

[ThorstenHans]

@tpmccallum please note:

I've moved both, the Vault Provider and the Azure Key Vault Provider sample apps to this location in fermyon/enterprise-architectures-and-patterns.

The most recent commit to this PR 0e5731e updates corresponding links

[tpmccallum]

Noticed the system never created a private preview, but it looks perfect on localhost preview: