Rewrite the README around a runnable quick start

[sadeqabuhattem]

Why

New users couldn't get started from the README:

• Install failed. Only 1.0.0-preview.1 is on NuGet, so dotnet add package DotNetBoost.Settings.Core errors with "There are no stable versions available".
• The Quick Start couldn't run. It used an AppDbContext it never defined, had no using lines, and needed EF migrations.
• Port 5000 fails on macOS. AirPlay Receiver holds it and answers 403.

What changed

• README.md is 870 → ~240 lines. It opens with a one-line pitch, then a 5-minute quick start: a single-file EF Core + SQLite Program.cs plus three curl commands that change a setting while the app runs. After that:
  • reading and writing settings in your own code
  • a table for plugging in your existing database
  • an "I want to…" table linking each feature to its guide
  • the one-command Aspire demo
  • the package list
• The anonymous-endpoints warning now sits right after the quick start instead of halfway down.
• Reference material moved to docs/, with nothing removed. There are 13 topic pages plus an index at docs/README.md. Relative paths and cross-page anchors are fixed.
• README links are absolute GitHub URLs, because the README is also the NuGet package readme (PackageReadmeFile) and relative links break on nuget.org.
• --prerelease is on every install command.
• docs/storage-providers.md now has the install commands and using lines each provider needs.
• docs/validation.md:
  • adds the DotNetBoost.Settings.FluentValidation install step
  • corrects the "every write path" claim: Data Annotations are only enforced by the REST POST, not by SetAsync(). The code change to fix that is a separate follow-up.

Verification

I ran the quick start exactly as written in a fresh dotnet new web project against the published 1.0.0-preview.1 packages:

• the first read returned smtp.example.com:587
• the POST returned 204
• the next read returned smtp.mycompany.com:2525
• after a restart, the value was still smtp.mycompany.com:2525

Every relative link under docs/ resolves. Note that nuget.org keeps showing the old readme until the next package is published.

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