Create v0.7.6 version of site

[nate-double-u]

Create v0.7.6 site, last release of the v0.7 set.

• [x] setup release-0.7.6 branch
• [x] bring new branch in line with https://github.com/containernetworking/plugins/tree/v0.7.6
• [x] setup netlify site (including v0-7-6.cni.dev url)
• [x] update other versions for dropdown menu.

[nate-double-u]

https://v0-7-6.cni.dev/ wired up, and content in line with https://github.com/containernetworking/plugins/tree/v0.7.6