fix: Include [Experimental] attribute support by default

[filzrev]

This PR intended to fix issue #10552

outputFormat: ApiPage supports ExperimentalAttribute (It's identified by name)
And render HTML with Preview badge with alert text.
Example: https://dotnet.github.io/docfx/seed/apipage/BuildFromProject.Class1.html

But when using System.Diagnostics.CodeAnalysis.ExperimentalAttribute attribute.
It's filtered out by default filterconfig settings.
Because when namespace is excluded. all types inside namespace are also excluded.

This PR modify defaultfilterconfig.yml rules and enable ExperimentalAttribute by default.

[filzrev]

Following snapshot file diffs seems be caused by PR(#10113)

test/docfx.Snapshot.Tests/SamplesTest.Seed/index.verified.json