Add #-based redirects to "About us" section

[yndajas]

When we moved from "Who we are" to "About us", I set up a bunch of Jekyll redirects. These work, but links like /who-we-are/#our-mission and /who-we-are/#make-everything-we-do-accessible will only redirect to /about-us (with the hash fragment) using this method

We can use the redirect manager to improve the experience with links like these, so that /about-us/#our-mission would redirect to /about-us/our-mission-values-and-principles/#our-mission and /about-us/#make-everything-we-do-accessible would redirect to /our-mission-values-and-principles/#make-everything-we-do-accessible, while still redirecting to the section landing page (/about-us) by default

TO DO

• set up redirects for all sections that existed in "Who we are" before #1148 that have direct replacements in the current "About us" section

Related: #1086