Add documentation for rockbox_coverart extension

docs/extensions/rockbox_coverart.md

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+---
+title: Rockbox Cover Art Sync
+---
+
+This extension writes podcast subscription cover art to a media player.
+This extension assumes that each podcast has its own folder on device.
+Only folder-wide coverart is written - this extension does not do anything about
+embedded cover art.
+
+Cover art is converted to the desired file format and image size
+via the python "pillow" module, and written to the desired filename.
+
+Existing files (of the correct name) will be checked to see if they
+need to be overwritten to conform to the size, baseline/progressive,
+and file format parameters.
+
+All jpegs will be written as "baseline" (rather than "progressive") type. Rockbox
+only supports baseline jpegs.
+
+## Dependencies
+
+Requires python modules `pillow` and `filelock`. These are available on pip.
+
+## Settings
+
+| setting | value | description |
+|---------|-------|-------------|
+| extensions.rockbox_coverart.art_name_on_device  | cover.jpg | Desired filename on device. Only jpg, jpeg, and png filetypes allowed. |
+| extensions.rockbox_coverart.convert_and_resize_art | **True**/False | Enable conversion and resize operations |
+| extensions.rockbox_coverart.convert_size | 500 | Cover art will be resized to this size (square). If convert_allow_upscale_art=False, art smaller than this size will retain its size. Only used if convert_and_resize_art=True. |
+| extensions.rockbox_coverart.convert_allow_upscale_art | True/**False** | Allow upscaling of art. If set to true, will allow images smaller than convert_size (square) to be upscaled to convert_size. Only used if convert_and_resize_art=True. |
+
+Links:
+
+- [Extension Source](https://github.com/gpodder/gpodder/blob/master/share/gpodder/extensions/rockbox_coverart.py)