simplify setup instructions

[rvhonorato]

This PR cleans and simplify the installation instructions to run a local copy of the website.

The instructions were a big convoluted, requiring you to manually create the dependency file - i simplified it by simply making the necessary modifications for it and adding the .lock file to avoid surprises

I am not super sure about the ruby installation, please double-check the command for other operating systems.

[AnnaKravchenko]

Is it normal to see this warning popping multiple times? Deprecation Warning: Using / for division outside of calc() is deprecated and will be removed in Dart Sass 2.0.0.
Is it’s smth general, or is happening specifically on my laptop?

[rvhonorato]

Yes there are lots of deprecation notices. I think the dependencies here have never been updated in more than 10 years, I'm surprised it still works. I tried adding new versions but it broke the html render - so lets hope these versions here are still accessible in the long term

[AnnaKravchenko]

I also see multiple ** ERROR: directory is already being watched! **.
Not pretty, but very much functionlal, nice!