The current version of ESPHome shows a warning about GPIO4:
```
WARNING GPIO4 is a Strapping PIN and should be avoided.
Attaching external pullup/down resistors to strapping pins can cause unexpected failures.
See https://esphome.io/guides/faq.html#why-am-i-getting-a-warning-about-strapping-pins
```
The warning can be ignored, so I added a remark about this to the documentation.
Thanks to @jossuanet for the heads up about this warning.

* New light transformer implementation
* Fixed missing ; in the code, thanks Jos!
* Removed backward compatibility code for v1.20.x (because there were too many other changes that broke the build, and for which creating a work-around was not feasible).
* Fixed duplicate api section in the example.yaml (thanks @badbroechten\!) and updated the example to not use an underscore in the hostname.
* Prepare for release

* Updated documentation for 1.18.0 release with external components support.

* Moved example.yaml to the root dir, in line with the recommendations from the ESPHome external components guide.

* Normalized the version numbering for ESPHome references.

* Fixed the styling of the example.yaml references.

Co-authored-by: Maurice Makaay <mmakaay1@xs4all.net>