Thermometer doc improvements: OTA, MQTT Discover, the RESET button, and some minor typography

[cederlys]

Improve the doc for ANAVI Thermometer:

• document recent changes to the OTA procedure
• document the new support for Home Assistant MQTT Discovery
• fix the documentation for the RESET button, and document recent improvements
• minor typographic end-of-chapter fixes

[leon-anavi]

Hi @cederlys,

Ops, sorry, I forgot about this PR. I will rebase and merged it in the coming days.

Thanks, Leon

[leon-anavi]

@cederlys merged, thank you!