Furo theme (#3490)

* Set up folder structure and stubs for beta doc site using furo theme.
Squash merge Includes:
- Fill out folder structure for all TOC sections. Complete AdaRound example.
- Make beta doc build optional for agile development.
- Stripped beta piccolo theme CSS down to only modifications of the stock theme CSS.
- Swap in furo theme for look & feel and right-side menu.
- Refine furo theme options.
- More mods to furo theme. Set up index for API ref.

Signed-off-by: Dave Welsch <dwelsch@expertsupport.com>

* Add copyright headers to snippet .py files to fix repolinter complaint.

Signed-off-by: Dave Welsch <dwelsch@expertsupport.com>

* Removed unnecessary templates, cleaned up CSS file.

Signed-off-by: Dave Welsch <dwelsch@expertsupport.com>

* Fix TOC to display 2 levels, no collapse.

Signed-off-by: Dave Welsch <dwelsch@expertsupport.com>

---------

Signed-off-by: Dave Welsch <dwelsch@expertsupport.com>

Docs/beta/_static/aimet-furo.css

@@ -0,0 +1,12 @@
+
+ aside.toc-drawer ul ul ul ul {
+   display: none;
+}
+
+.toctree-l3 {
+    display: none;
+}
+
+[for^="toctree-checkbox-"], [id^="toctree-checkbox-"] {
+    visibility: hidden;
+}