Mkdocs-rss-plugin

Fixed

* wrong items order in updated feed

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Fixed

* feed for updated content is broken - 15

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Added

* plugin: add the `language` tag to the channel
* tooling:
* add first unit tests and code coverage
* add Github Action to perform tests
* check docker build using mkdocs-material

Changed

* docs: switch Feedly images from HTTP to HTTPS

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Fixed

* remove print from plugin code

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Added

* plugin: add tag `guid` to the feed's items (using the page URL)

Changed

* docs: minor improvements

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Added

* plugin: handle channel `image` and items (entries) `enclosure`

Changed

* plugin: refactoring to build feed entry through a method