Docs: how peer builder features are already solved the Simple Builders way

[AndreasIgel]

Documentation: how peer builder features are already solved the Simple Builders way

Closes #225.

Why

A feature comparison against Lombok, Immutables, AutoValue, FreeBuilder and RecordBuilder found that several capabilities those libraries advertise are already supported by Simple Builders — but implemented in an SB-specific way that follows its design paradigm (augment the types you already have; never redefine their semantics). Because the "SB way" of each wasn't documented, a fresh comparison (human or AI) mistakes them for missing features. This PR is documentation only — no source or behavior changes — so those "gaps" aren't re-raised.

What changed (README only)

"How Simple Builders compares" → new subsection "Doing what other builders advertise — the Simple Builders way", with one concise entry per capability, each linking to the relevant Usage section:

• Required fields / null-safety
• Copy / with / toBuilder (the With interface)
• Collection immutability (the target type's responsibility)
• Incremental / "singular" collection API (add2X, collection builders, varargs)
• Inheritance (inherited setters + subclass-constructor-exposed fields)

(Default values are intentionally not listed — SB has no first-class default mechanism; that missing feature is tracked in #227.)

Plus an explicit note that generating value semantics (equals/hashCode/toString) and brand-new immutable value types are deliberate out-of-scope paradigm choices, not missing features.

Usage section — two new subsections documenting inherent behavior (placed here, not in CONFIGURATION.md, because these are behaviors, not configurable options):

• Required Fields and Null-Safety (after Validation Annotations) — a field is non-nullable when its type is primitive or it carries a NotNull/NonNull-named annotation (any package, matched by simple name). build() then enforces: constructor fields must be set and non-null; setter fields, if set, must be non-null — violations throw IllegalStateException. Includes a record example.
• Collection Immutability (end of Collections and Nested Objects) — immutability of built collections is the target type's responsibility; SB stores exactly what the type stores and never silently wraps collections. Includes a record compact-constructor List.copyOf(...) example.

TOC updated for all three new subsections. docs/CONFIGURATION.md is unchanged.

Verification

• git diff --check clean
• mvn -N validate passes
• All new internal anchors resolve against actual headings

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

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