Revert "Add subprojects/gi-docgen to Files-Excluded"

Revert "Add subprojects/gi-docgen to Files-Excluded"

Until we actually remove it (by importing an upstream version without
it), Policy says we need to keep its entries in d/copyright.

gi-docgen upstream also still says gi-docgen is not stable, and still
recommends vendoring it, so keep it included until that changes.

This reverts commit 93ed2478d3e12b429fe75b9e414d26906e04f17c.