MT#55897 RTD: fix sphinx based theme

It's been noticed the previous default (static)
binding has been broken, hence explicitely define
usage of `sphinx_rtd_theme` and set appropriate
versioning requirements.

Change-Id: I847f7f3d0128fc2269449b7487524c8cd57e36b3

docs/conf.py

@@ -14,7 +14,8 @@ release = 'master'
 # -- General configuration ---------------------------------------------------
 # https://www.sphinx-doc.org/en/master/usage/configuration.html#general-configuration
 
-extensions = ["myst_parser"]
+extensions = ["myst_parser",
+              "sphinx_rtd_theme"]
 myst_heading_anchors = 3
 
 templates_path = ['_templates']
@@ -25,8 +26,5 @@ master_doc = 'index'
 # -- Options for HTML output -------------------------------------------------
 # https://www.sphinx-doc.org/en/master/usage/configuration.html#options-for-html-output
 
-html_theme = 'default'
-html_static_path = ['_static']
-
+html_theme = 'sphinx_rtd_theme'
 htmlhelp_basename = 'rtpenginedoc'
-

docs/requirements.txt

@@ -1 +1,2 @@
 myst_parser
+sphinx_rtd_theme>=1.2