docs: fix broken links, stale content and housekeeping - #11839

Summary

Documentation cleanup in docs/ and one small change to a generator script. No runtime code changes.

Broken links

• Getting-started.md (no such page) now points to Installation.md in Examples, Tutorials, the connector tutorials, the SOAP tutorial and migration/mounting-lb3app.
• Calling-other-APIs-and-Web-Services.md now points to Accessing-services.md, which is the target the published site redirects to.
• Database-connectors.md now points to Database-connectors.html, a sidebar-only page.
• ./apidocs/*.md links now use the published apidocs.<pkg>.<symbol>.html form. I checked these against loopback.io.
• Migration pages: wrong relative paths, extensions.md and docs/site/... links.
• After these changes, a local script finds no unresolved relative .md links under docs/site. The repository.defaultcrudrepository* API pages return 404 on loopback.io today. They are generated by npm run tsdocs, so I kept them in the same form as the other API links.

Housekeeping

• Renamed Create-your-first-app copy.md to Create-your-first-app.md. The permalink is unchanged, so the published URL stays the same.
• MONOREPO.md had no front matter. I added it, and updated bin/update-monorepo-file.js so regeneration keeps it. I didn't run the script because dependencies aren't installed here, so I added the same header by hand.
• Moved the four *.shelved.md pages from docs/site to docs/spikes/shelved.

Stale content

• The hosted SOAP calculator (calculator-webservice.mybluemix.net) is down. This is also noted in FIXMEs in examples/soap-calculator. The tutorial now has a warning, and the generic page uses a placeholder URL. I didn't change the example project.
• Replaced apidocs.strongloop.com links.
• Resolved (TBD) entries in Model.md, Understanding-the-differences.md and REST-action-sequence.md. Where a feature doesn't exist I wrote "Not available yet" rather than inventing content. Please review the wording.
• Upgraded http:// to https:// for loopback.io, expressjs.com, passportjs.org, graphql.org and sailsjs.com. I confirmed all five serve HTTPS.

Not done

• Pinned-commit links (Binding.md, Controller.md, Model.md and others) are unchanged. They still resolve.
• The Bluemix/IBM Cloud deployment guides still reference bluemix.net. They need a separate rewrite.
• Commented-out TODO/FIXME blocks are left as they are.
• I haven't built the Jekyll site.

Checklist

• DCO signed in all commits
• npm test passes on your machine (docs only, not run)
• New tests added or existing tests modified to cover all changes (n/a)
• Code conforms with the style guide (prettier checked on all changed files)
• API Documentation in code was updated (n/a)
• Documentation in /docs/site was updated
• Affected artifact templates in packages/cli were updated (n/a)
• Affected example projects in examples/* were updated (n/a)