Add secretgen-controller documentation to the website

[a-mccarthy]

A sub-issue of #744.

Currently, secretgen-controller does not have its docs set on the website. The website currently points to the secretgen-controller GitHub repo. This is a poor user experiences as it takes readers out of the website. As secretgen-controler is a part of carvel, it should be equally part of the website like the other projects.

We should do the work to add the seretgen-controller docs to the website. This will include:

• Setting up a new landing page for secretgen-controller on the website
• Moving over the /docs folder to a new secretgen/docs folder here https://github.com/carvel-dev/carvel/tree/develop/site/content
• Updating the secretgen-controller README to point readers to the site for docs, similar to have other projects do it, https://github.com/carvel-dev/kapp-controller/tree/develop/docs.
• We also need to add any site data for secretgen-controller including a table of contents https://github.com/carvel-dev/carvel/tree/develop/site/data

Open questions:

• How many versions of the secretgen-controller docs do we want to have on the website?

[a-mccarthy]

there may be other areas that need to be updated, but these are what comes to mind first.

[github-actions]

This issue is being marked as stale due to a long period of inactivity and will be closed in 5 days if there is no response.