Add examples of calling two algorithms to README

[natebosch]

Closes #103

Reword most of the README.

• Use "callback" consistently to describe the arguments to the
  algorithms.
• Add generics and more specific names to the tiny example data
  structures.
• Use consistent phrasing in the bullet points about the types of
  callbacks needed.
• Reorder the bullet points since the edges callbacks are always used
  and worth mentioning first.

Add a code block with skeleton examples of calls using each of the
example data structures. One example of calls shortestPath on a graph
with adjacency lists stored in a Map using terms related to
networking. The other calls topologicalSort on a graph represented by
a tree of node objects which store outgoing edges using terms related to
build dependencies.

[github-actions]

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Breaking changes ✔️

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Package	Change	Current Version	New Version	Needed Version	Looking good?
graphs	None	2.3.3-wip	2.3.3-wip	2.3.3-wip	✔️

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The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

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// Copyright (c) 2024, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

Files
no missing headers

All source files should start with a license header.

Package publish validation ✔️

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Package	Version	Status
package:cli_config	0.2.1-wip	WIP (no publish necessary)
package:extension_discovery	2.0.1-wip	WIP (no publish necessary)
package:graphs	2.3.3-wip	WIP (no publish necessary)
package:unified_analytics	6.1.3-wip	WIP (no publish necessary)

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[devoncarew]

lgtm, but note the two diagnostics from the unintended_html_in_doc_comment lint