Add initial ChemicalJSON documentation migrated from archived wiki

[Shruthii7]

This PR adds an initial docs/ directory with a Markdown overview of the
ChemicalJSON format, migrated from the archived Open Chemistry wiki.

It also updates the README to point users to the in-repo documentation
while a dedicated documentation site is being considered.

[Shruthii7]

Thanks for pointing me to the archived wiki page.

I’ve migrated the core ChemicalJSON documentation into docs/overview.md and linked it from the README, adapting the content into Markdown and organizing it for easier maintenance.

Happy to refine or expand this further if you’d like additional sections or changes.