docs: align setup guides with current Control Plane UI

[walker-tx]

Summary

• Update the canonical Speakeasy setup template and 21 published guides for current navigation, catalog completion, and remote-server connectivity verification.
• Handle authentication state, existing versus new providers/clients, automatic discovery, callback verification before attachment, and read-only reused-client scopes.
• Correct catalog-time header entry and preserve provider-specific credentials, links, and newer Salesforce guidance on main.

Evidence and scope

• UI source audit against speakeasy-api/gram main at 8fa18729608e34de305e789b53f36eb2c6c853c9; not a production-browser walkthrough.
• Records the approved doctrine change in the changelog.
• Generated Go content was regenerated for validation but is intentionally excluded from this guide PR. The normal post-merge regeneration/release workflow remains responsible for publication.
• Manual fallback issuer/endpoint evidence is still incomplete for GitHub, NetSuite, Salesforce, and Snowflake. Those paths explicitly require documented administrator-supplied values instead of guessing.

Validation

Passed in an isolated worktree based on current main:

• All guide directories: go run ./cmd/lint-guide ../guides/*/
• Generator: go run . and go test .
• Module, guide linter, and guide-check tests and vet
• git diff --check

Validation used installed Go 1.27.1 because local Go 1.22 hit a macOS LC_UUID loader failure; no toolchain configuration was changed.