docs(admin): update installation wizard to match current implementation

[joshtrichards]

☑️ Resolves

The installation wizard documentation was outdated relative to the current server implementation (i.e. after nextcloud/server#51464). This updates the page to accurately reflect the current state.

• Changed UI and terminology
• Missing wizard features now documented
• Database user creation rewritten for (near) accuracy
  • It's better than it was; currently working in nextcloud/server on unifying some minor differences in the implementations
• Clarified Trusted domains:
  • Correct loopback documentation: localhost, 127.0.0.1, and [::1] are
    all whitelisted via isTrustedDomain() regardless of trusted_domains
    config (was previously only mentioning 127.0.0.1)
  • Document wildcard pattern support
• Also fixes some grammar
• Updated a couple of the screenshots

• But I confess I was feeling lazy at that point and did it after the fact by using the ones from the implementation PRs...

Follow-up (probably separately):

• Review all the images; update cleanly in a single pass to bring all up-to-date

🖼️ Screenshots