docs: improve API examples organization

[wujinhh]

Summary

• reorganize the API documentation around the README's operator groups
• add missing built-in utility type examples for Record, Parameters, and ConstructorParameters
• add missing examples for Partial, Readonly, ReturnType, and InstanceType
• fix a mismatched ValuesType example label and clarify several sample type names

Verification

• git diff --check HEAD~1..HEAD

Closes #51

[gemini-code-assist]

Code Review

This pull request updates the README.md to document several built-in utility types (such as Record, Parameters, and ConstructorParameters), adds subheadings to better organize the document, and provides usage examples for existing types. The review feedback highlights structural mismatches and ordering inconsistencies between the Table of Contents and the document body, as well as inconsistent labeling of built-in types in their section headers.

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[gemini-code-assist]

The Table of Contents is missing DeepNonNullable<T>, which is documented in the body under #### Deep object utilities (around line 958). Additionally, there is a structural mismatch between the Table of Contents and the actual document body:

1. The H2 heading ## Special operators exists in the Table of Contents (line 135) but is completely absent in the body, where its types are grouped under H4 subheadings under ## Object operators.
2. The order of types in the Table of Contents (e.g., listing Deep utilities under ## Object operators before Pick/Omit) does not match the actual order in the body (where Deep utilities are placed near the end, after PromiseType).

We should add DeepNonNullable<T> to the Table of Contents and align the TOC structure/order with the actual body organization.

[gemini-code-assist]

There is an inconsistency in how built-in types are labeled in their section headers:

• Omit, Record, Parameters, and ConstructorParameters have _(built-in)_ appended to their H3 headers (e.g., ### Omit<T, K> _(built-in)_).
• Partial, Readonly, ReturnType, and InstanceType do not have _(built-in)_ in their H3 headers (e.g., ### Partial``).

To maintain consistency across all built-in utility types, we should either append _(built-in)_ to all of their headers (and update their Table of Contents anchors accordingly) or keep the headers clean and only use the _(built-in)_ label in the Table of Contents.