docs: simplify API and provider guidance

[Hou (SciencePotato)]

Summary

• Rebuild docs/CONTRACT.md as a compact Microsoft Learn-style Send OTP API reference.
• Make docs/BRING-YOUR-OWN-PROVIDER.md a focused Add Twilio guide. Twilio is not bundled or configured by guided setup.
• Move built-in Soprano and Telesign behavior, provider settings, credential caching, JWE validation, runtime differences, and production readiness into TECHNICAL.md.
• Simplify the root README and documentation index with task-based tables and direct links to setup, troubleshooting, monitoring, API, runtime, and advanced guidance.
• Rework monitoring, Application Insights, sign-in monitoring, onboarding, deployment, cost, logging, Front Door, and fallback guidance with short task indexes, direct steps, compact tables, and focused reference sections.
• Refine phrasing across the landing page, technical reference, operational guides, and runtime READMEs so instructions are direct, natural, and consistent.
• Focus the Front Door guide on customer setup, capacity, security, recovery validation, and rollback. Remove historical trial data and timing comparisons while keeping the documented routing limits.
• Keep Front Door and provider fallback detailed because they are advanced implementation references, while shortening their entry points and navigation.
• Update runtime and operational pages to link to one canonical home instead of repeating provider, monitoring, and readiness details.
• Preserve exact API fields, settings, algorithms, validation reasons, provider status rules, authentication boundaries, runtime behavior, retry safeguards, and delivery semantics.

Measured reduction

This PR reduces all documentation from 43,886 to 30,116 whitespace-delimited Markdown words, a net reduction of 13,770 (31.4%) from the merged #54 content.

The final structure and language pass reduces the previous PR55 head from 38,271 to 30,116 words, removing another 8,155 words (21.3%).

Page	Previous PR55 head	Final	Change
docs/MONITORING.md	4,503	1,895	-2,608
docs/SIGN-IN-MONITORING.md	4,141	1,579	-2,562
docs/ONBOARDING.md	3,056	1,753	-1,303
docs/APPLICATION-INSIGHTS.md	2,255	1,219	-1,036
docs/FRONTDOOR.md	4,153	3,600	-553
All documentation	38,271	30,116	-8,155

The short operational pages now lead with the reader's task. Detailed protocol, Front Door, fallback, and built-in provider behavior remain in their canonical references.

Validation

• Checked all 19 Markdown pages: 496 local links, 276 anchors, and 14 image references passed.
• Rendered all 19 pages at 1280px and 390px. All 38 page and width combinations passed with no broken images or page-level overflow.
• Passed 20 click-through preview journeys.
• Confirmed the Twilio guide does not mention built-in provider names.
• Confirmed required API, setting, error, provider, runtime, monitoring, identity, delivery, and cryptography terms remain in the canonical references.
• Retained 45 focused command, code, query, and HTTP examples.
• Confirmed no PNG changed and git diff --check passed.

Local previews approximate GitHub styling. External URL availability was not checked. No runtime suite, live query, Azure sign-in, deployment, provider send, paid action, or repository tooling change was needed for this documentation-only pass.

Integration

This is a five-commit follow-up to merged #54. It does not import commits from #53 or duplicate #50 hosting-cost work.

#53 overlaps TECHNICAL.md, runtime READMEs, and setup guidance. Any later integration should preserve the short API reference, the Twilio-only guide, and TECHNICAL.md as the home for built-in provider details.