Add libraries.md for .NET 11 Preview 2 (#10267)

* Add libraries.md for .NET 11 Preview 2

* Improve libraries-release-notes skill and copilot instructions

- Add reviewer suggestion step (Step 5) that gathers PR authors, assignees,
  mergers, and coauthors to suggest release notes reviewers grouped by area
- Add feature grouping guidance to categorize related PRs under single sections
- Add positive tone guidelines to editorial rules
- Add issue/PR reference format rules: {org}/{repo}#{number} with markdown links
- Add copilot-instructions.md entry so future sessions discover the skill
- Update data collection and enrichment steps to populate reviewers table
- Exclude bots and Copilot from all reviewer suggestions including coauthors

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Add .NET 11 Preview 2 Libraries release notes

New features: ProcessExitStatus, PosixSignal.SIGKILL, TarFile format
selection, and JsonSerializerOptions.GetTypeInfo<T>().

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address feedback and augment .NET Libraries Preview 2 release notes.

Author release notes for .NET Libraries in .NET 11 Preview 2 covering
the 2026-01-26..2026-02-11 date range. Includes:

- Generic GetTypeInfo<T> for System.Text.Json
- Tar archive format selection
- Matrix4x4.GetDeterminant ~15% faster
- Bug fix summary across 9 areas
- Community contributors section

Process APIs (ProcessExitStatus, PosixSignal.SIGKILL) excluded per
instructions as the feature is incomplete.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Delete libraries-release-notes skill. Use the release-notes skill from #10282 instead.

* Reformat bug fixes section to use grouped sub-bullet style

Update the Libraries Preview 2 release notes bug fixes section to use
nested sub-bullets grouped by area, matching the revised formatting
guidelines. Multi-fix areas (System.Net.Http, System.Numerics) are now
consolidated under their area heading, and single-fix areas also use
sub-bullets for consistency.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Jeff Handley <jeffhandley@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 3 people