Polish pass on design tokens documentation

[mkernohanbc]

When we have some bandwidth, I'm going to do a bit of a refresh on the tokens documentation — particularly the front matter/introductory page.

Specifically looking at:

• Better interlinking to core tokens documentation on gov.bc.ca
• Clarifying messaging around how we handle releases, and what kind of change users can expect from future releases

I think we can do more to reassure people that this library is intended to be relatively stable, and that major changes that will force refactoring/relearning are going to be rare.

Earmarking this for a potential 3.0.1 or 3.1.0 release, along with #350.

[mkernohanbc]

Docs updates staged on v3.0.1 branch in Figma.

[mkernohanbc]

Completed with 3.1.1 update, rolling out today/tomorrow.