Add ESP32 example to "Build Sample Application" section of Firmware docs

[szczys]

There is currently a tab for this but it is empty. I'm taking it out until we can populate it. This issue should help us remember.

Relevant code that I'm removing:

• https://github.com/golioth/docs/blob/eddd0e0b62930ba5422d19fd739bd8b26fe86894/docs/firmware/device-sdk/firmware-upgrade/2-build-sample-application.md?plain=1#L20
• https://github.com/golioth/docs/blob/eddd0e0b62930ba5422d19fd739bd8b26fe86894/docs/firmware/device-sdk/firmware-upgrade/2-build-sample-application.md?plain=1#L61-L63

[szczys]

There is now a sample application section for ESP-IDF in the getting started guide: https://docs.golioth.io/getting-started/device-examples/compile-example-code/esp-idf/flash-sample