doc: match link text in index to doc headline

The index ToC says "About these docs" but the document itself says
"About this documentation" which I think is better. Use that.

PR-URL: https://github.com/nodejs/node/pull/34449


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>