Add docs for Überzug++ as image previewer (kamiyaa#359)

* Add docs for Überzug++ as image previewer

Adding instructions how Überzug++ can be used to preview image files in
Joshuto.

Additionally:
* Restructure image preview documentation into multiple files in a
  sub-directory.
* Add deprecation note and remove dead link to installation instructions
  for the Überzug recipe.
* Some text enhancements here and there.

* rename readme.md to README.md

Author: DLFW

docs/README.md

@@ -4,6 +4,6 @@ These docs should help you get set up and understand how Joshuto works.
 
 - [command line arguments](/docs/command_arguments.md)
 - [configuration](/docs/configuration/)
-- [image previews](/docs/image_previews.md)
+- [image previews](/docs/image_previews/README.md)
 - [misc](/docs/misc.md)
 - [contributing](/docs/contributing.md)

docs/image_previews.md

@@ -0,0 +1,58 @@
+# Image Thumbnails in File Previews
+
+Joshuto does not support image previews directly.
+One reason is that Joshuto wants to stay independent of specific display protocols and terminal emulators.
+
+However, Joshuto offers two preview-related hooks which allow to easily implement an
+image preview with some simple scripts.
+The hooks can be configured in the `joshuto.toml` file.
+```toml
+[preview]
+...
+preview_shown_hook_script = "~/path/to/some/executable_script_1"
+preview_removed_hook_script = "~/path/to/some/executable_script_2"
+```
+The shown-hook is called whenever a new file-preview (in the 3rd pane) is activated.
+
+The removed-hook will be called each time the file preview panel
+completely disappears in Joshuto.
+That is the case if the user selects a file for which no file preview is shown
+(either due to missing output of the preview script or due to file size),
+if the preview is not cached already and the preview pane is temporarily removed,
+or if the selection jumps from a file to a directory.
+
+The “shown” script gets the path of the file being previewed
+as first argument and then the x and y coordinate and the width an
+height of the preview area as second to fifth parameters.
+
+The “removed” script does not get any arguments.
+
+Using these hook, one can trigger various actions when moving the cursor along files in Joshuto,
+and they can also be used to show image previews by the help of other 3rd party tools.
+
+Keep in mind that the result of the “normal” `preview` script you use for textual previews
+is cached by Joshuto and is not called every time a file is focused, but the “shown” hook is.
+
+# Wrapper Script
+For some of the 3rd party tools, it's necessary 
+to run them as a separate process, in parallel to Joshuto.
+
+One famous example is “Überzug”. To be able to use such a solution,
+one need to have a “wrapper-script” which must be started instead of Joshuto.
+The wrapper-script will then start both, first the program for showing images
+in the terminal and then Joshuto.
+
+
+# Recipes
+## Image Thumbnail Solution Recipes
+We have recipes for a few famous solutions.
+* [Überzug](ueberzug.md) (only for X11)
+* [Überzug++](ueberzugpp.md) (for X11, some Wayland compositors, and some specific terminal emulators)
+* [Kitty](kitty.md) (for the Kitty terminal)
+
+## Other Recipes and Tricks
+* [Combining text preview and image preview](combined_with_text.md)
+
+# Recipe Contributions welcome 🤗
+
+Feel free to provide recipes to include in this documentation.