Replace front-page "learn more" link with hard-coded 2.19 one (#171)

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](https://www.pantsbuild.org/community/members#how-can-i-contribute-to-pants)
points to `/2.20/...`, which is probably reasonable for a contributing
guide, but less so for a "getting started" one):
https://github.com/pantsbuild/pantsbuild.org/blob/ac1d7051581bf4be77abd95b0405aa97202e6aad/src/pages/community/members.mdx#L28

src/pages/_index.mdx

@@ -116,7 +116,7 @@ You'll find no subsets like Starlark here. Pants empowers you with full support
 
 Pants supports Python, Docker, Go, Java, Kotlin, Pex, Protodoc, Scala, Shell, Thrift, Protobuf,
 Docker, Helm, many linting and formatting tools, packaging, coverage, and more.
-[Learn more.](/docs/introduction/welcome-to-pants)
+[Learn more.](/2.19/docs/introduction/welcome-to-pants)
 
 </div>
 </div>