doc: add tsc-agenda to onboarding labels list

PR-URL: #32832
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

Author: Trott

doc/guides/onboarding-extras.md

@@ -21,6 +21,8 @@ request.
 * `feature request` - Any issue that requests a new feature (usually not PRs)
 * `good first issue` - Issues suitable for newcomers to process
 * `meta` - For issues whose topic is governance, policies, procedures, etc.
+* `tsc-agenda` - Open issues and pull requests with this label will be added to
+  the Technical Steering Committee meeting agenda
 
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