Adding Stuart Mumford to the Documentation Infrastructure team

[Stuart Mumford]

I propose to add myself to the role documentation infrastructure role,
this is to support my work rolling out and then maintaining a unified
theme for the Astropy website and coordinated package documentation (see
https://github.com/astropy/astropy-project/issues/522).

See https://www.astropy.org/team for a list of current roles. Following
APE 0 [1] we leave a two week long comment period for this proposal
where you can comment publicly on the PR [2] or privately to the CoCo
(coordi...@astropy.org or DM one of them on Slack). If there are no
objections after two weeks, the CoCo shall merge the PR [3].

Sincerely,
Stuart

[1] https://github.com/astropy/astropy-APEs/blob/main/APE0.rst
[2] https://github.com/astropy/astropy.github.com/pull/705
[3] Technically, APE0 only requires the two-week long comment period for
additions, but following the same spirit, the CoCo also invites comments
for removal unless the person in a role explicitly requested to be
removed or did not respond to consecutive role surveys.