docs/about, docs/extension-point: fix broken links

Linking to .md or .html works either way when used from a Gerrit
instance, but linking to .md makes the link work when browsing the
document in Gitiles.

The libModule link in extension-point.md was just wrong. This change
makes the link work when used in a Gerrit instance.

Change-Id: I44806523cb8e4aec8393c1fcc36aba4a78c37e66