Adds doc instructions on how to import

[petems]

A small docs note on how to import vault_jwt_auth_backend, as I made the assumption it'd be the path and had to look at the code to figure it out! 😅

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Relates OR Closes #0000

Release note for CHANGELOG:

Add import section to documentation for the `vault_jwt_auth_backend`

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccXXX'

...

[jasonodonnell]

Thanks!