Notebooks: not handling YML-relative paths

[patridge]

When a notebook URL is relative to the current .yml file, the extension isn't handling it correctly.

Example: https://docs.microsoft.com/en-us/learn/modules/intro-machine-learning-keras/3-neural-network Source (snapshot): https://github.com/MicrosoftDocs/learn-pr/blob/dadac869491ac698f5b63cf245b06506843ae882/learn-pr/tensorflow/intro-machine-learning-keras/3-neural-network.yml#L15

notebook: notebooks/3-neural-network.ipynb

[patridge]

In the extension, we seem to only allow the relative form starting at the repo level. https://github.com/patridge/learn-metadata-tool/blob/4a2fbd059eeaada96dbe1d0d5d58f2a6f138fc17/azure-devops-extension-popup.js#L155

Looks like we need to handle links relative to current content similarly.