Switch to ex_doc for generating documentation

[fhunleth]

I tried to make the smallest amount of changes to the README.md. They were mostly to fix Markdown to render better with ex_doc. It looks like this:

[Copilot]

Pull request overview

This PR migrates the documentation system from edoc to ex_doc for the cerlc library, which generates CRC (Cyclic Redundancy Check) functions in Erlang.

• Adds rebar3_ex_doc plugin configuration to rebar.config
• Removes the old doc/overview.edoc file that was used by edoc
• Updates README.md with improved formatting, minor fixes, and documentation generation instructions
• Updates .gitignore to ignore the /doc directory instead of individual edoc artifacts

Reviewed changes

Copilot reviewed 2 out of 4 changed files in this pull request and generated 3 comments.

File	Description
rebar.config	Adds rebar3_ex_doc as a project plugin with configuration for source URL, main page, and extras
doc/overview.edoc	Removes the entire edoc overview file as part of migration to ex_doc
README.md	Reformats content, fixes Elixir example syntax, and adds new section explaining how to generate docs with ex_doc
.gitignore	Updates to ignore /doc directory instead of individual edoc-info file

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[fhunleth]

Sorry for the spam. I forgot that Copilot would auto-review this PR when I made it.