Upgrade hugo and docsy version (add dark theme)

[milinddethe15]

• Latest docsy release has lightdark-mode-menu which maybe fix Implement Dark/Light Mode Compatibility for Website #3692.
• This will remove lots of warnings logs, add latest features, bug fixes and other improvements.

[thesuperzapper]

We run a highly custom version of docsy theme, see the overrides in the layouts/ folder, so it will probably be a very large amount of work.

It might even be easier to make our own light/dark mode, but I am not sure because this may require updating all the pages and make our overrides even worse, we should check how they did it on the upstream docsy repo, and see if it was just a change in the common page templates.

[milinddethe15]

google/docsy#1909 added the dark mode feature in docsy.

I will give a try to implement this here.

[milinddethe15]

/assign @milinddethe15

[thesuperzapper]

I have created a PR that will add a dark theme, while retaining the hugo/docsy version as doing the upgrade would have been nearly impossible given our customizaions to the theme:

• Add dark theme to website #3981