docs: Add optional sphinx_rtd_theme; add docs build instructions.

The sphinx_rtd_theme is used by ReadTheDocs to render a pretty looking
documentation.  If you have this theme installed locally then your
locally-compiled docs will look exactly like the published
documentation.   Otherwise it falls back to the default theme.

docs/conf.py

@@ -103,9 +103,17 @@ pygments_style = 'sphinx'
 
 # -- Options for HTML output ----------------------------------------------
 
-# The theme to use for HTML and HTML Help pages.  See the documentation for
-# a list of builtin themes.
-html_theme = 'default'
+# on_rtd is whether we are on readthedocs.org
+on_rtd = os.environ.get('READTHEDOCS', None) == 'True'
+
+if not on_rtd:  # only import and set the theme if we're building docs locally
+    try:
+        import sphinx_rtd_theme
+        html_theme = 'sphinx_rtd_theme'
+        html_theme_path = [sphinx_rtd_theme.get_html_theme_path(), '.']
+    except:
+        html_theme = 'default'
+        html_theme_path = ['.']
 
 # 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
@@ -113,7 +121,7 @@ html_theme = 'default'
 #html_theme_options = {}
 
 # Add any paths that contain custom themes here, relative to this directory.
-html_theme_path = ['.']
+# html_theme_path = ['.']
 
 # The name for this set of Sphinx documents.  If None, it defaults to
 # "<project> v<release> documentation".
@@ -154,7 +162,7 @@ html_last_updated_fmt = '%d %b %Y'
 
 # Additional templates that should be rendered to pages, maps page names to
 # template names.
-html_additional_pages = {"index":"topindex.html"}
+html_additional_pages = {"index": "topindex.html"}
 
 # If false, no module index is generated.
 #html_domain_indices = True
@@ -234,7 +242,7 @@ latex_documents = [
 # (source start file, name, description, authors, manual section).
 man_pages = [
     ('index', 'micropython', 'Micro Python Documentation',
-     ['Damien P. George'], 1)
+     ['Damien P. George'], 1),
 ]
 
 # If true, show URL addresses after external links.