Add doc fetch actions

[Confectrician]

This will add a reusable action base for fetching external docs contents.

Goal

Provide updated "stable" docs for parts of openHAB, that have their own indipendent release cycle.

Workflow

Reusable template action

The introduced reusable action is meant to be called from other repo based actions.
It will check for matching files based on a given configuration and remove the current existing doc files.
After that it will copy the newest available version from the external repository.

Detailled repository action

A detailed action can be added per external repository.
This action simply calls the reuseable action with a repository specific configuration.
As a proof of concept the first repository based action comes with this pr and enables this workflow for the openhab/openhabian repository.

External trigger events

The detailled repository action is able to get triggered from the external repo through the repository_dispatch event.
This needs a personal access token, with public_repo access for the openhab-docs repository.

The external repo can then actively trigger a docs update. (For example when a new openHABian version is released.)

[netlify]

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