Prepare 0.7.0, README update

[dhardy]

I have pencilled in tomorrow as the release date, but it may slip (holiday plans).

[Copilot]

🟡 Not ready to approve

There are a few documentation correctness issues (typo, stale/broken links, and a future-dated changelog entry) that should be fixed before tagging the release.

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Pull request overview

Prepares the easy-cast crate for the 0.7.0 release by updating the crate version and refreshing user-facing documentation (README + crate docs + changelog) to reflect the newer rounding-mode-based API.

Changes:

• Bump crate version to 0.7.0.
• Refresh README and crate-level docs with updated positioning, trait overview, and examples.
• Add a new 0.7.0 changelog entry describing the rounding-mode API revision.

File summaries

File	Description
src/lib.rs	Updates crate-level rustdoc introduction formatting and emphasis.
README.md	Rewrites README to match the newer API model (Cast/Conv, CastApprox/ConvApprox, CastTo/ConvTo) and adds quick example + motivation sections.
CHANGELOG.md	Adds a 0.7.0 release notes section summarizing new rounding traits/modes and API changes.
Cargo.toml	Bumps package version to 0.7.0.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 2
• Review effort level: Lite

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