docs: Fix issue in readthedocs locating theme.

Error:
    NameError: name 'html_theme' is not defined

Also fixes a couple of issues highlighted by the sphinx build
output.

docs/conf.py

@@ -37,6 +37,7 @@
     'sphinx.ext.mathjax',
     'sphinx.ext.ifconfig',
     'sphinx.ext.viewcode',
+    'sphinx_rtd_theme',
 ]
 
 # Add any paths that contain templates here, relative to this directory.
@@ -339,16 +340,9 @@
 # If false, no index is generated.
 #epub_use_index = True
 
-
-# Example configuration for intersphinx: refer to the Python standard library.
-intersphinx_mapping = {'http://docs.python.org/': None}
-
 # on_rtd is whether we are on readthedocs.org, this line of code grabbed from docs.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
-    import sphinx_rtd_theme
-    html_theme = 'sphinx_rtd_theme'
-    html_theme_path = [sphinx_rtd_theme.get_html_theme_path()]
+html_theme = 'sphinx_rtd_theme'
 
 # otherwise, readthedocs.org uses their theme by default, so no need to specify it

docs/index.rst

@@ -93,7 +93,6 @@ Projects related to MicroPython on the BBC micro:bit include:
    :maxdepth: 2
    :caption: Developer Guide
 
-   devguide/installation
    devguide/flashfirmware
    devguide/repl
    devguide/hexformat

docs/neopixel.rst

@@ -28,6 +28,7 @@ art and games such as the demo shown below.
 
 .. image:: neopixel.gif
    :alt: Neopixel flexible tile
+
 Image attribution: `adafruit flexible Neopixel matrix <https://www.adafruit.com/product/2547>`_
 
 To connect a strip of neopixels you'll need to attach the micro:bit as shown

docs/requirements.txt

@@ -0,0 +1,2 @@
+sphinx<8
+sphinx_rtd_theme<2