[CI/infra] Reduce site build time by 60% by setting sidebar_cache_limit to 1

[chalin]

Learning from our sister project kubernetes/website, I'd like to propose that we set the following config parameter (reference):

[params.ui]
sidebar_cache_limit = 1 # https://github.com/gohugoio/hugo/issues/8918#issuecomment-903314696

In local tests, doing so reduces the site build time by 60%. The cost is that the active side-nav entry corresponding to the current page is dynamically marked as active client-side via the JS in https://github.com/google/docsy/blob/fed2d28d247f4d9f1190a5088f0fea4f09676b84/layouts/partials/sidebar.html (that JS should be moved, but that's another story).

WDYT @open-telemetry/docs-maintainers?

Note that this is blocked on:

• Language dropdown menu on mobile: paths default to locale homepage not translations of current page google/docsy#2131

/cc @sftim @nate-double-u