Clarify Default includes and excludes section in .NET project SDK overview

[Copilot]

The "Default includes and excludes" section was vague about what each item type does, didn't explain the glob column semantics, and left the "Remove glob" column unexplained—leading users to incorrectly assume they should add these globs to their project files.

Changes

• Opening paragraph → bulleted list: Now explicitly describes what Compile, EmbeddedResource, and None items do (compiled into assembly / embedded in assembly / tracked but not compiled or embedded).
• Glob terminology defined before the table: New bullet list defines include/exclude/remove glob so the table columns are self-explanatory.
• None's remove glob explained: Added a paragraph after the table clarifying that None uses **/* as its include, so the SDK removes **/*.cs and **/*.resx to prevent those files from appearing in both None and their primary item types simultaneously.
• Warning promoted to [!WARNING] admonition: The original plain-text sentence was easy to miss. Rewritten as a [!WARNING] block with a clearer message: the SDK already includes these items—don't define them again.

Original prompt

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This section details on the original issue you should resolve

<issue_title>The Default Compilation Includes section is confusing and possibly misleading</issue_title>
<issue_description>An example of what some of these permutations would translate to in a proj file would probably help clarify.

Like what does the <None ... element's "Remove Glob" actually do?

This also implies that globbing syntax can be used in a proj file but Im not seeing that actually working.

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Document Details

⚠ Do not edit this section. It is required for docs.microsoft.com ➟ GitHub issue linking.

• ID: aa484235-d872-f50e-96fc-aa5b4a2a42fa
• Version Independent ID: 871c22e0-65a2-f3a4-41b9-f050489dba7a
• Content: .NET project SDK overview
• Content Source: docs/core/project-sdk/overview.md
• Product: dotnet-fundamentals
• GitHub Login: @gewarren
• Microsoft Alias: gewarren</issue_description>

<agent_instructions>The opening paragraph in this section could use more detail on what these items do for your project, and the callout for not modifying them could be stronger.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes The Default Compilation Includes section is confusing and possibly misleading #21753

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Internal previews

📄 File	🔗 Preview link
docs/core/project-sdk/overview.md	.NET project SDKs

[BillWagner]

This is ready for final review.

[Copilot]

Pull request overview

Improves the Default includes and excludes section in the .NET project SDK overview to reduce confusion about what the SDK implicitly includes, how the glob table should be read, and why None has a remove glob.

Changes:

• Replaces the prior opening paragraph with a bulleted explanation of Compile, EmbeddedResource, and None item types.
• Adds pre-table definitions for include, exclude, and remove glob semantics.
• Adds a post-table explanation for None’s remove glob, and upgrades the duplicate-items guidance to a [!WARNING] admonition.