feat: update NFS migration skill with operator config, missing blueprints, and entity page changes

[rohitkrai03]

Fixes

• https://redhat.atlassian.net/browse/RHIDP-14477

Summary

Fills 30 content gaps in the NFS migration skill reference files, identified by comparing against redhat-developer/rhdh#5009 (the official RHDH migration guides). Key additions:

• New operator-config.md — app.extensions resolution rules, app.routes.bindings, app.packages, operator cheat sheet (legacy → NFS task mapping), config merging semantics
• 10 missing blueprints — SearchResultListItemBlueprint, SearchFilterBlueprint, FormFieldBlueprint, AddonBlueprint, ThemeBlueprint, SignInPageBlueprint, IconBundleBlueprint, NavContentBlueprint, PluginWrapperBlueprint, EntityContextMenuItemBlueprint
• 6 missing mount point mappings — entity.context.menu, search.page.*, application/header, updated appIcons
• Entity page structural changes — tab groups, type: info/type: content layout, built-in extension IDs, Dependencies tab removal, system diagram change, API path change
• Scalprum dual-export documentation — scalprum.name, exposedModules, module/importName resolution
• 8 new gotchas (chore: rename create-skill to skill-maker #15-22) — config merging, entity page changes, RHDH-specific gaps, operator limitations
• Extended verification checklist — blueprint-type-specific checks, scalprum compatibility, OCI rebuild
• Upstream Backstage doc links in support.md
• Removed docs/nfs-migration-guide.md — reference files now stand alone; official human-readable docs will live in the rhdh repo

Test plan

• pytest — 343 pass (1 pre-existing failure in test_update_base_image)
• ruff check — clean
• Zero dangling references to deleted guide
• All 15 reference files exist and are indexed in SKILL.md
• All 10 new blueprint names appear in at least one reference file

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