Update chisel index page (#497)

[edenizk]

Styling update for the static Chisel index page. Closes #497

[luboskmetko]

@edenizk this should actually generate a list of pages like described here https://www.getchisel.co/docs/development/pages :)

I think <!-- CHISEL-PAGES-LIST --> was a placeholder used for that. Also when I run add-page command I'm getting this error. The template is created, though. Maybe that's just related to my local linking of packages, but please verify that it works without an error. Thanks

TypeError: Cannot read property '1' of null
    at Command.<anonymous> (C:\xampp\htdocs\www\test\chisel-499\node_modules\chisel-plugin-static-frontend\lib\commands\add-page.js:160:64)
    at async Promise.all (index 0)
    at async Service.run (C:\xampp\htdocs\www\test\chisel-499\node_modules\chisel-scripts\lib\Service.js:185:5)
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

[edenizk]

@luboskmetko, Fixing the placeholder fixed the problem thank you 🙂
I have also added styling for the code element which was missing in the preview I was working on

[luboskmetko]

Thanks, merged this.