docs(dotnet): document https-only URL validation and AllowInsecureUrls

[dobby-coder]

Closes #78.

Updates the .NET docs to reflect the URL validation added in encryption4all/postguard-dotnet#15.

Changes

• docs/sdk/dotnet-encryption.md: note under the constructor explaining that PkgUrl and CryptifyUrl must be absolute https:// URLs and that the constructor throws ArgumentException otherwise. Added an AllowInsecureUrls row to the options table.
• docs/repos/postguard-dotnet.md: short comment beside the constructor snippet mirroring the same rule.

Verification

• npm run docs:build passes (5.7s).
• Built HTML present at docs/.vitepress/dist/sdk/dotnet-encryption.html and docs/.vitepress/dist/repos/postguard-dotnet.html.

Reviewer quickstart

```
git fetch origin && git checkout docs/dotnet-https-validation && npm install && npm run docs:dev
```