add Institutional RSE Networking affinity group

[jsubida]

Description

Added a link to Institutional RSE Networking page in the Affinity Groups page

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[cmaimone]

@jsubida I think this PR is missing the page (institutional-rse-networking.md) it's trying to link to as a part of it, which should be put in https://github.com/USRSE/usrse.github.io/tree/main/pages/ag and the URL would be

{{ site.baseurl }}/ag/institutional-rse-networking/

Note that the first - in the current file should be a / instead

[jdamerow]

@cmaimone I thought the same thing at first too, but then I realized the link is simply linking our AG site at https://us-rse.org/ag-institutional-rse-networking/ 😅 Or do you prefer to have a separate AG page on the US-RSE site and link to our AG blog from there?