Add a new Demo projects for beginners

[muzir]

Add a new Demo project which help beginners to see Themeleaf on PROD and also check the code.

[danielfernandez]

Hello @muzir, thanks a lot for your PR.

Unfortunately, the Demos section of the ecosystem page is one that is bound to be soon removed, as there are already enough general example applications at the official sets of examples, Spring guides, etc. The one entry right now there is basically there because we didn't know where to put the test dialect for direct SQL querying we created at the team… but we will need to move it elsewhere, probably to the dialects section.

The idea of the ecosystem page is to have a sort of registry of extensions, integrations and tools, but to be honest right now it is even a bit of a mess given some of the community dialects listed there aren't updated yet to 3.0, so a cleanup will probably soon follow: thymeleaf/thymeleaf-dist#5

Thanks anyway!