ci: drop TravisCI and ReadTheDocs, replaced by Github Actions and mkdocs

docs/configuration.md

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+# Configuration files
+
+Guessit supports configuration through configuration files.
+
+Default configuration file is bundled inside guessit package from
+[config/options.json][] file.
+
+It is possible to disable the default configuration with
+`--no-default-config` option, but you have then to provide a full
+configuration file based on the default one.
+
+Configuration files are loaded from the following paths:
+
+> -   `~/.guessit/options.(json|yml|yaml)`
+> -   `~/.config/guessit/options.(json|yml|yaml)`
+
+It is also possible to disable those user configuration files with
+`no-user-config` option.
+
+Additional configuration files can be included using the `-c`/`--config`
+option.
+
+As many configuration files can be involved, they are deeply merged to
+keep all values inside the effective configuration.
+
+# Advanced configuration
+
+Configuration files contains all options available through the command
+line, but also an additional one named `advanced_config`.
+
+This advanced configuration contains all internal parameters and they
+are exposed to help you tweaking guessit to better fit your needs.
+
+If no `advanced_config` is declared through all effective configuration
+files, the default one will be used even when `--no-default-config` is
+used.
+
+We're willing to keep it backwards compatible, but in order to enhance
+Guessit, these parameters might change without prior notice.
+
+  [config/options.json]: https://github.com/guessit-io/guessit/blob/develop/guessit/config/options.json/