Refresh README and examples to present petgraph-decypher as a Cypher query/mutation engine

[Copilot]

• Review new PR comment and confirm required scope
• Verify current README wording around Cypher trademark/compatibility language
• Update README phrasing to use Cypher-compatible wording without endorsement implications
• Run targeted validation for the documentation-only change
• Run final review/security validation and reply to the PR comment

[Copilot]

Pull request overview

This PR refreshes the public-facing documentation and example code to present petgraph-decypher as a Cypher front-end for petgraph that can build graphs, execute read queries, and apply mutations to existing graphs.

Changes:

• Reframed README.md from “builder-only” to “build + query + mutate”, including a capability matrix and a limitations/roadmap table.
• Added README examples demonstrating querying and mutating an existing petgraph::Graph, plus pointers to typed/custom-weight support.
• Extended examples/indiana_jones.rs to demonstrate cypher_mut (SET), richer projections (DISTINCT, ORDER BY), scalar functions, and OPTIONAL MATCH.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
README.md	Updates positioning, adds capability/limitations tables, and expands usage examples for querying/mutating existing graphs.
examples/indiana_jones.rs	Enhances the example to showcase mutations and richer read-query semantics (functions, distinct/order, optional match).

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