"Learn more" link on front page is hard-coded to 2.19, should be latest stable

[huonw]

In #171 / pantsbuild/pants#20648, we adjust the front page "getting started" link to not be dead, by hard-coding it to the current latest stable series: 2.19.

Once we release 2.20, this link should point to the /2.20/... page.

---

We have one page that computes a dynamic link using the latest version:

pantsbuild.org/src/pages/community/members.mdx

Line 28 in ac1d705

We try and make contributions easy. For example, you can suggest documentation fixes by clicking on the Suggest Edits link on any page. And you can report bugs by opening a [GitHub issue](https://github.com/pantsbuild/pants/issues). If you want to hack on the Pants codebase itself there is a <Link to={`/${versions[0]}/docs/contributions`}>helpful guide</Link>.

import { default as versions } from "@site/versions.json";
import Link from "@docusaurus/Link";

... <Link to={`/${versions[0]}/docs/contributions`}>helpful guide</Link>.

That links to the latest non-dev version (which could be an RC pre-release) not the latest stable version, so we may want to find a better way to do it (and wrap it up in a component? https://docusaurus.io/docs/markdown-features/react).

Preferably we retain link validity checking for it too (#66).

[huonw]

One way to address this might be to have a custom field storing the latest stable version, that components can access via useDocusaurusContext. This would centralise the logic around "stable versions" etc. to all happen in docusaurus.config.js.