Remove deprecated line highlight feature for Rehype in docs

[slevithan]

Should the Rehype line highlight feature be removed from the docs? It currently says the following:

Warning

This is deprecated. It's disabled by default in v0.10.0 and will be removed in the next minor. Consider use transformerNotationHighlight instead.

This note is now quite old.

[nyxsky404]

I've opened a PR to remove the deprecated line highlight section from the docs as requested.
PR: #1172