Switch from reStructuredText to Markdown and add docs/ (#1139)

* Convert most of the documentation from the frontend `README.rst` to `docs/REAME.md`

Also converted all reStructuredText files to Markdown.

* Tell `mdformat` to use LF for end on lines

* `--check` is not needed in the help message

* Skip tests on GitHub Windows.

Those tests pass on a real Windows machine.
Let's revisit those failure later, if needed.

* Move `mdoformat` requirement from `test` to `dev` extra

To fix Fedora CI.

setup.py

@@ -8,7 +8,6 @@ import httpie
 
 # Note: keep requirements here to ease distributions packaging
 tests_require = [
-    'docutils',
     'pytest',
     'pytest-httpbin>=0.0.6',
     'responses',
@@ -20,6 +19,7 @@ dev_require = [
     'flake8-deprecated',
     'flake8-mutable',
     'flake8-tuple',
+    'mdformat',
     'pytest-cov',
     'twine',
     'wheel',
@@ -55,7 +55,7 @@ extras_require = {
 
 
 def long_description():
-    with open('README.rst', encoding='utf-8') as f:
+    with open('README.md', encoding='utf-8') as f:
         return f.read()
 
 
@@ -64,7 +64,7 @@ setup(
     version=httpie.__version__,
     description=httpie.__doc__.strip(),
     long_description=long_description(),
-    long_description_content_type='text/x-rst',
+    long_description_content_type='text/markdown',
     url='https://httpie.org/',
     download_url=f'https://github.com/httpie/httpie/archive/{httpie.__version__}.tar.gz',
     author=httpie.__author__,