Replace front-page "learn more" link with hard-coded 2.19 one

[huonw]

This papers over the dead "Learn more" link on the pantsbuild.org front page (pantsbuild/pants#20648) by replacing it with the hard-coded /2.19/docs/introduction/welcome-to-pants, for the current latest-stable version.

This isn't a full solution: preferably this would be a "dynamic" link, pointing to the latest stable version. Thus this doesn't close that issue.

The dynamic approach we take in some other pages isn't perfect because it isn't to the latest stable release, but can include pre-releases (it currently points to /2.20/..., which is probably reasonable for a contributing guide, but less so for a "getting started" one):

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>.

[benjyw]

Fine to stop the bleeding, but yeah, we should figure out a "latest stable version" thing for these. Thanks!