doc: sphinx: Load RTD theme as a Sphinx extension

This commit loads the RTD theme as a Sphinx extension, which has the
benefit of going through said extension's "setup" method, effectively
setting useful settings such as default permalink icon (moving away from
the default, not so pretty, "¶").

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>

doc/conf.py

@@ -8,8 +8,6 @@ import re
 import textwrap
 
 from sphinx.cmd.build import get_parser
-import sphinx_rtd_theme
-
 
 args = get_parser().parse_args()
 ZEPHYR_BASE = Path(__file__).resolve().parents[1]
@@ -69,6 +67,7 @@ release = version
 
 extensions = [
     "breathe",
+    "sphinx_rtd_theme",
     "sphinx.ext.todo",
     "sphinx.ext.extlinks",
     "sphinx.ext.autodoc",
@@ -136,7 +135,6 @@ rst_epilog = """
 # -- Options for HTML output ----------------------------------------------
 
 html_theme = "sphinx_rtd_theme"
-html_theme_path = [sphinx_rtd_theme.get_html_theme_path()]
 html_theme_options = {
     "logo_only": True,
     "prev_next_buttons_location": None