Add license details

[MakisH]

Closes #247 by implementing the suggestions stated in #247 (comment)

• Adds more details in the "Licenses" section of README.md, clarifying the license of the website and dependencies.
  • These include Jekyll, the theme, and the libraries fetched in head.html.
• Adds SPDX headers to all _plugins, attributing to The preCICE developers - precice.org, using the years I could retrieve from the history.
  • Similarly to the files in js/ and tools/ that were written specifically for the preCICE website and might be reusable.
• Removes the navgoco license, since that has been removed in Migrate from Bootstrap 3.3.7 to Bootstrap 5.3.8 #922.

I probably have missed a few files or dependencies, but this should already be much clearer than before.

FYI @fsimonis @MuhammadAashirAslam @uekerman

This is also relevant and will need to be adapted for Hugo (#979).

[fsimonis]

My approval for the record