docs: explain how to use Primitives alongside System.Reactive

[glennawatson]

Summary

The README now explains how to use ReactiveUI.Primitives in a project where another package still brings in System.Reactive.

• New section "Using both libraries in one project" under "Moving from System.Reactive". It lists three fixes for the ambiguous Subscribe call (error CS0121): an alias on the System.Reactive package reference for the whole project, a using directive inside the namespace for one file, and SubscribePrimitives for one call.
• The note near the top of the README now links to that section instead of naming only SubscribePrimitives.

Why

Users who add ReactiveUI.Primitives next to a package that depends on System.Reactive hit CS0121 on every Subscribe call.

• The previous note only offered SubscribePrimitives, which means changing every call site.
• The package alias fixes a whole project with one line in the project file or Directory.Build.props.
• Related to [Bug]: Ambiguity between ReactiveUI.Primitives and System.Reactive extension methods #154.

Breaking changes

None. Documentation only.

How this was verified

Every snippet in the new section compiles against the current build in a project that references System.Reactive.

• That includes plain .Subscribe(...) calls resolving to ReactiveUI.Primitives once the alias is in place.

Notes for the reviewer

The whole change is in README.md.

• The section sits under the migration guide, since the repository rules keep System.Reactive references under the comparison and migration headers.

Checklist

• I have read the Contribute guide
• The PR title follows Conventional Commits
• Tests cover this change, or the summary says why they do not
• New or changed public API has XML documentation

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.78%. Comparing base (2a8b66d) to head (fe3c282).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

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  Coverage   99.78%   99.78%           
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  Files         785      785           
  Lines       24850    24850           
  Branches     2946     2946           
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  Hits        24796    24796           
  Misses         54       54           

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