ci: deploy package index to esp8266.github.io #6062
                
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https://github.com/esp8266/esp8266.github.io repository is the source of a Github Pages site, visible as https://esp8266.github.io and https://arduino.esp8266.com.
Previously, when arduino.esp8266.com was hosted on a custom server, Travis CI used to run a deploy script, which pushed the new package_esp8266com_index.json over SSH.
Now that https://arduino.esp8266.com is generated from https://github.com/esp8266/esp8266.github.io, we need to modify that. Deploy script will clone the Github Pages repository, replace the package index, commit the changes, and push them to GitHub.