docs: refactor CNI plugin content (#23707)

- Pulled common content from multiple pages into new partials
- Refactored install/index to be OS-based so I could add linux-distro-based instructions to install-consul-cni-plugins.mdx partial. The tab groups on the install/index page do match and change focus as expected.
- Moved CNI overview-type content to networking/index
- Refactored networking/cni to include install CNI plugins and configuration content (from install/index).
- Moved CNI plugins explanation in bridge mode configuration section into bullet points. They had been #### headings, which aren't rendered in the R page TOC. I tried to simplify and format the bullet point content to be easier to scan.

Ref: https://hashicorp.atlassian.net/browse/CE-661
Fixes: #23229
Fixes: #23583

website/content/docs/concepts/plugins/cni.mdx

@@ -61,4 +61,4 @@ Nomad Networking documentation](/nomad/docs/networking/cni).
 [cni_spec]: https://www.cni.dev/docs/spec/
 [cni_spec_net_config]: https://github.com/containernetworking/cni/blob/main/SPEC.md#configuration-format
 [cni_spec_plugin_config]: https://github.com/containernetworking/cni/blob/main/SPEC.md#plugin-configuration-objects
-[nomad_install]: /nomad/tutorials/get-started/get-started-install#post-installation-steps
+[nomad_install]: /nomad/tutorials/get-started/get-started-install#linux-post-installation-steps