Fix citation volume and dead featuretools link

[godarrenw]

What

Two small documentation fixes:

1. Corrects the journal volume in the Teh et al. (2021) citation — IEEE Sensors Journal 15.16 → 21.16 — in both places it appears (README.md:67 and docs/text/introduction.rst:105).
2. Updates the dead featuretools link in docs/text/introduction.rst:72.

Why

Citation volume. Crossref returns the following for 10.1109/JSEN.2021.3084970, which is the DOI already linked next to the citation:

title:     Expect the Unexpected: Unsupervised Feature Selection for
           Automated Sensor Anomaly Detection
container: IEEE Sensors Journal
volume:    21
issue:     16
page:      18033-18046
issued:    2021-08-15
authors:   Teh, Wang, Kempa-Liehr

Title, authors, issue and page range all match what is written; only the volume differs. IEEE Sensors Journal was on volume 21 in 2021, so 15.16 looks like a slip for 21.16.

featuretools link. https://www.featuretools.com/ no longer resolves — the TLS certificate does not match the host, and forcing past it with curl -k returns 404. The bare domain fails the same way. The project's documentation now lives at https://featuretools.alteryx.com/ (200, redirecting to /en/stable/).

The other two links in that sentence — FATS and cesium — both still return 200 and were left untouched.

How I checked

The volume was verified against the Crossref API rather than inferred, and the link states were taken with curl -L. I also checked the neighbouring reference [6] (10.1016/j.ascom.2021.100460, Astronomy and Computing 35.100460): Crossref gives volume 35 and page 100460, which matches what is written, so it was left alone.

Documentation only — no code changes.

Disclosure: prepared with AI assistance; the citation metadata and both link states were verified manually.