Minor fixes

[vincerubinetti]

Running list of minor things to fix:

• Why are on-pull-request and on-push .yml instead of .yaml? I vaguely recall github inconsistently requiring .yml vs .yaml in different cases, but I'm not sure this is one of them. .yaml is canonically what you're supposed to use according to YAML, so I've tried to always use that.
• Add flex-shrink: 0 to _styles/header.scss .logo and width: unset
• Add <details> styling.
• Update docs to note that in periodic citation updates, when we automatically open a pr, we can't automatically trigger a PR preview build.