Add a custom not-found page

[TimHess]

Adds a 404 page to source and nginx config

[github-actions]

Preview link: https://docs-steeltoe-pr-357.azurewebsites.net

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az webapp config appsettings set --resource-group steeltoe --name docs-steeltoe --slot pr-<this-PR-number> --settings mainsite_host=https://www-steeltoe-pr-<other-pr-number>.azurewebsites.net

This is an automated message.

[bart-vmware]

If we can't reuse the existing Razor component to render the menu, I think we should leave it out. This copy easily gets out of sync, resulting in more 404s. Causing a 404 on the 404 page is embarrassing.

The return link should just point to "https://docs.steeltoe.io/", which redirects to whatever version is current at that moment.

[TimHess]

I almost left a note in the description but figured I'd mention it at standup today... My first attempt here was to add a .md file so that we'd get everything in the docfx template, but the first attempt also came with the v4/v3/v2 menu. I was already planning to revisit this ASAP, during the docfx update