[HELP] SSI usage with netutils/thttpd

### Description

I am successfully using the thttpd library to serve my pages and run custom cgi scripts but I have not managed to coerce it to parse the index.html page for simple SSI directives such as date.

As best as I can find out - and I appreciate this is not "native" nuttx-apps and is quite old - I should be able to get it to parse by changing the usual URL `http:/10.0.0.1/index.html` to a URL like this: `http:/10.0.0.1/cgi-bin/ssi/index.html` (which is where the cgi "applications" are located, albeit virtually I think)

But I might be missing the point?

It's not a big deal as I can sort this using my own CGI script, in the style of a REST API or something, but it annoys me that I can't get it to work!

### Verification

- [x] I have verified before submitting the report.

Reproduce the SSI request using the index.html and cgi-bin/ssi/index.html paths described in the issue, then inspect the thttpd SSI and CGI entry points. Determine whether the date directive is supported and document the expected URL or the source change and test needed to make the behavior work.

Written by the indexing model from the issue text.