Add SIG links to Links (#2656)

* create `sigs` directory under links
* create `sigs/index.md`
* create `links/.pages` to properly format Sigs to SIGs

Author: sspencerwire

docs/rocky_insights/links/.pages

@@ -0,0 +1,5 @@
+ nav:
+    - ... | index*.md
+    - SIGs: sigs
+    - ...
+

docs/rocky_insights/links/sigs/index.md

@@ -0,0 +1,10 @@
+---
+title: Special Interest Groups
+author: Steven Spencer
+contributors:
+---
+
+* Alternate Architecture [SIG/AltArch](https://sig-altarch.rocky.page)
+* Cloud Services [SIG/Cloud](https://sig-cloud.rocky.page/)
+* Infrastructure [SIG/Infra](https://infra.rocky.page/)
+* Release Engineering [SIG/Core](https://sig-core.rocky.page)