Move docs back to root, move onionshare_gui into briefcase app, and make modifications so briefcase app will work

docs/README.md

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+# OnionShare Documentation
+
+All these commands must be run from the `docs` folder.
+
+You must have the python dependencies installed to build the docs:
+
+```sh
+poetry install
+```
+
+To build HTML docs:
+
+```sh
+poetry run make html
+```
+
+Then open `docs/build/html/index.html` in a browser to see it.
+
+To update all of the translations and build all the html, run:
+
+```sh
+poetry run ./build.sh
+```
+
+You will end up with the documentation in all supported locales in `docs/localized_html`.
+
+Here's how preparing translations works:
+
+```sh
+# Generate .po files in build/gettext 
+make gettext
+
+# Create a new blank German locale in source/locale, based on .po files
+sphinx-intl update -p build/gettext -l de
+
+# Build German translated document
+make -e SPHINXOPTS="-D language='de'" html
+```