Add a README per example and an index in the root README

[tillig]

Part of #32

Proposed Changes

• A README per example. Fifteen directories under src/ and no way to tell them apart without opening the source. Each now says what it demonstrates, which packages it needs, how to run it, and which documentation page it follows. Four or five lines each, not a design document.
• An index table in the root README, grouped into ASP.NET Core, hosting and core features, and .NET Framework, so the repo is browsable from the top.
• Examples.slnx groups projects by area instead of a single flat src folder.

The src directory layout is deliberately unchanged. Moving directories would break all ten inbound links from the Documentation repo and gains nothing once the table exists.

Two small judgment calls. The three ASP.NET Core examples share one doc page and differ only in hosting shape, so each README says how it differs from its siblings rather than repeating the same paragraph three times. And ConfigurationExampleInterface and ConfigurationExamplePlugin get READMEs that say they are supporting libraries and point at ConfigurationExample, since they'd otherwise look like examples someone forgot to document.

Verification

Every src directory has a README, and all 15 projects are still in the solution — I diffed the project set before and after regrouping rather than trusting the file to look right.

All 32 markdown links are checked programmatically. Relative links are resolved against the filesystem. The documentation links are validated against the Documentation repo by mapping each readthedocs.io/en/latest/<path>.html back to docs/<path>.rst and confirming the file exists, which catches a wrong page name without needing the network.

The run instructions were executed rather than assumed. Both /api/values examples bind http://localhost:5000 and return ["value1","value2"], and the child lifetime scope example serves ApplicationA on 5000 and ApplicationB on 5001. I checked each log for Hosting failed too, because a stale listener from an earlier run can answer curl and make a failed start look like a pass.

dotnet build Examples.slnx still succeeds, and pre-commit including markdownlint passes.