Add the total numbers of members and alumni

[yotamofek]

My partner asked me roughly how many people are "official" rust team members, and I couldn't say, so thought it might be a good addition to the "all team members" page.

Not sure about the placement and whether others will think it's a good idea too.

r? @Kobzol as the person who added this page in the first place :)

[rustbot]

Failed to set assignee to Kobzol: invalid assignee

Note: Only org members with at least the repository "read" role, users with write permissions, or people who have commented on the PR may be assigned.

[Kobzol]

I was also thinking that displaying the count would be nice. 👍