Update variables documentation with latest APIs and better example

[kate-goldenring]

Our variables example showed a bad example of checking passwords -- it does not at minimum do a constant time comparison. While it is an example, we cannot assume that folks will not use it. Instead, this updates the example to be a more realistic one that calls an API with a token.

This also updates the examples to use the latest SDKs -- Python was out of date -- and talks about templating within the manifest.

I've run all examples locally -- here is a repo with the examples for if we need to test them for future sdk changes: https://github.com/kate-goldenring/spin-variables-examples

Pre-Merge Content Checklist

This documentation has been checked to ensure that:

• [ ] The title, template, and date are all set
• [ ] 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)
• [ ] File does not use CRLF, but uses plain LF (hint: use cat -ve <filename> | grep $'\r' | wc -l and expect 0 as a result)
• [ ] Has passed bart check
• [ ] Has been manually tested by running in Spin/Bartholomew (hint: use PREVIEW_MODE=1 and run npm run styles to update styling)
• [ ] Headings are using Title Case
• [ ] Code blocks have the programming language set to properly highlight syntax and the proper copy directive
• [ ] Have tested with npm run test and resolved all errors
• [ ] Relates to an existing (potentially outdated) blog article? If so please add URL in blog to point to this content.

[itowlson]

I think all the CI failures are changes to the JS SDK reference which will get fixed via #1355

[kate-goldenring]

@itowlson this should be ready for another review