[hotfix][docs] Change how-to-contribute empty link to overview page

[swap-10]

The how-to-contribute link on the what is flink page currently links to /how-to-contribute/ This is an empty page. This commit changes the link to /how-to-contribute/overview/ which seems to be the intended helpful page.

I considered using an alias instead if possible, but the /what-is-flink URL also currently opens an empty page (no content), except that it's not linked to from anywhere so user experience is not impacted. So I assume this was a conscious design decision and just changed the link in question from /how-to-contribute/ to /how-to-contribute/overview/

I have built it locally with Hugo and tested that it works. Thanks!