doc: consider mentioning when rust 1.0 was released

[salewski]

The opening paragraph in the "What are Editions?" section currently states:

The release of Rust 1.0 established "stability without stagnation" as a core Rust deliverable. Ever since the 1.0 release, the rule for Rust has been that once a feature has been released on stable, we are committed to supporting that feature for all future releases.

It would be nice to note that Rust 1.0 was released in May 2015, since it answers a reader's next likely question, "When was that?"

Writing in Q3 2022, noting the date also helps establish the project's credible commitment to the rule. It's not something that was just thought up recently -- the project has been adhering to it for more than seven years.