Add secretgen-controller documentation to the website

A sub-issue of https://github.com/carvel-dev/carvel/issues/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?

Start by comparing the existing project pages and site data under site/content and site/data, then review the secretgen-controller /docs folder and README against the linked kapp-controller example. Done means a secretgen-controller landing page, migrated docs, site data including a table of contents, and README links to the website; clarify the desired documentation versions first.

Written by the indexing model from the issue text.