chore: general repo maintenance and docs cleanup (#3030)

Refresh README/docs for the current stack, add a security policy,
note the external-PR pause, and remove dead config and workflows.

.github/workflows/first-interaction.yml

@@ -1,13 +1,10 @@
 name: 'Welcome New Contributors'
 
 on:
-  pull_request:
-    types: ['opened']
   issues:
     types: ['opened']
 
 permissions:
-  pull-requests: write
   issues: write
 
 jobs:
@@ -20,10 +17,7 @@ jobs:
       - uses: actions/first-interaction@v1
         with:
           repo-token: ${{ secrets.GITHUB_TOKEN }}
-          pr-message: |
-            Thank you for creating your first Pull Request and for being a part of the open signing revolution! 💚🚀
-            <br /> Feel free to hop into our community in [Discord](https://documen.so/discord)
           issue-message: |
             Thank you for opening your first issue and for being a part of the open signing revolution!
-            <br /> One of our team members will review it and get back to you as soon as it possible 💚
+            <br /> One of our team members will review it and get back to you as soon as possible 💚
             <br /> Meanwhile, please feel free to hop into our community in [Discord](https://documen.so/discord)