Improve website documentation sections

[pleshakov]

(1) Move this page https://docs.nginx.com/nginx-gateway-fabric/how-to/traffic-management/integrating-cert-manager/ into a dedicated Traffic Security section, as it will better fit it, rather than Traffic Management

(2) Move https://docs.nginx.com/nginx-gateway-fabric/how-to/maintenance/upgrade-apps-without-downtime/ into Traffic Management, because the current “Maintenance and Upgrades” reads more like Maintenance and Upgrades of NGF, rather than the backend apps, which is what the guide covers: it is still about Traffic management, because it deals with Traffic management when upgrading those backend apps.

Acceptance

• Integrating cert manager doc fixes typo "Cert Manage"
• Upgrade-apps-without-downtime doc is moved to a Traffic Security Section
• New security section?

[github-actions]

This issue is stale because it has been open 14 days with no activity. Remove stale label or comment or this will be closed in 14 days.

[ADubhlaoich]

These suggestions seem completely reasonable to me, though I don't think the creation of a new section makes sense for a single standalone document.

There are some accidental instances of this already. We try to minimise the amount of "hops" a reader has to go through, so we avoid creating whole sections for single documents - moving them into a new section later if or when necessary is preferred.

I can take this issue on, if there's no objection.