doc: move all css files to _static/css

Move all CSS files to the _static/css subfolder.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>
diff --git a/doc/_static/zephyr-custom.css b/doc/_static/css/zephyr-custom.css
similarity index 100%
rename from doc/_static/zephyr-custom.css
rename to doc/_static/css/zephyr-custom.css
diff --git a/doc/conf.py b/doc/conf.py
index fbe306c..1ebf72e 100644
--- a/doc/conf.py
+++ b/doc/conf.py
@@ -579,7 +579,7 @@
 linkcheck_anchors = False
 
 def setup(app):
-    app.add_css_file("zephyr-custom.css")
+    app.add_css_file("css/zephyr-custom.css")
 
     app.add_js_file("https://www.googletagmanager.com/gtag/js?id=UA-831873-47")
     app.add_js_file("js/ga-tracker.js")