Exclude docc files from sources to avoid build warnings

[omochi]

When using this package as a dependency, the following warning appears during the build process:

warning: 'swift-algorithms': found 1 file(s) which are unhandled; explicitly declare them as resources or exclude from the target
    /Users/omochi/<myproject>/.build/checkouts/swift-algorithms/Sources/Algorithms/Documentation.docc

This happens because docc files are treated as undefined source files.
This patch explicitly excludes them using the exclude directive, clarifying that they are not source files and preventing the warning from being displayed.

Relations

• Warning for Documentation.docc: found 1 file(s) which are unhandled; explicitly declare them as resources or exclude from the target #245