doc: add `make html-live` and `html-live-fast`

Add two new commands that automatically locally host the generated
documentation upon completion and rebuild/rehost when the input files
change without any user interaction.

For more info see: https://pypi.org/project/sphinx-autobuild/

Signed-off-by: Jordan Yates <jordan@embeint.com>
diff --git a/doc/requirements.txt b/doc/requirements.txt
index 468be69..aa77efa 100644
--- a/doc/requirements.txt
+++ b/doc/requirements.txt
@@ -9,6 +9,7 @@
 sphinx-copybutton
 sphinx-togglebutton
 sphinx-sitemap
+sphinx-autobuild
 
 # YAML validation. Used by zephyr_module.
 PyYAML>=6.0