doc: use sphinxdoc HTML theme

Default 'classic' theme [1] was used by Python 2 documentation. It's not
well-suited for modern resolutions and it's sidebar width isn't
configurable, which is crucial for modules API ToC correct display.

1. https://www.sphinx-doc.org/en/master/usage/theming.html

Part of #67

Author: DifferentialOrange

Diff for: CHANGELOG.md

@@ -138,6 +138,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Bump msgpack requirement to 1.0.4 (PR #223).
   The only reason of this bump is various vulnerability fixes,
   msgpack>=0.4.0 and msgpack-python==0.4.0 are still supported.
+- Change documentation HTML theme (#67).
 
 ### Fixed
 

Diff for: doc/_static/tarantool.css

@@ -93,16 +93,16 @@
 
 # -- Options for HTML output ---------------------------------------------------
 
-html_style = 'tarantool.css'
+#html_style = 'style.css'
 
 # The theme to use for HTML and HTML Help pages.  See the documentation for
 # a list of builtin themes.
-html_theme = 'default'
+html_theme = 'sphinxdoc'
 
 # Theme options are theme-specific and customize the look and feel of a theme
 # further.  For a list of options available for each theme, see the
 # documentation.
-#html_theme_options = {}
+html_theme_options = {'sidebarwidth': '30%'}
 
 # Add any paths that contain custom themes here, relative to this directory.
 #html_theme_path = []
@@ -126,7 +126,7 @@
 # Add any paths that contain custom static files (such as style sheets) here,
 # relative to this directory. They are copied after the builtin static files,
 # so a file named "default.css" will overwrite the builtin "default.css".
-html_static_path = ['_static']
+#html_static_path = ['_static']
 
 # If not '', a 'Last updated on:' timestamp is inserted at every page bottom,
 # using the given strftime format.