docs: add deprecation notice

[jpeterson]

Description

This PR adds a deprecation notice to the docs and has no effect on the npm package.

Related Issue

Motivation and Context

The docs page still comes up often in search results and needs to indicate deprecation in addition to the README.

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.