Update dependency ex_doc to v0.39.1

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
ex_doc (source)	dev	minor	0.38.4 -> 0.39.1

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Release Notes

elixir-lang/ex_doc (ex_doc)

v0.39.0

Compare Source

• Enhancements
  • Allow custom search engines to be configured with support for https://hexdocs.pm
  • Improve admonition blocks so they better integrate with the page flow
  • Bug fixes
    • Add .cheatmd to EPUB to avoid broken links
  • Backwards incompatible changes
    • Validate :extras fields: if you were previously setting them to unexpected values, you may now get an exception
    • Setting exdoc:full-text-search-url metadata is no longer supported, using the new search engines configuration

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