Remove ignore from doctests

Documentation tests have some code snippets (examples) marked with ignore. The documentation specifies this is not the best option:

The ignore directive tells Rust to ignore your code. This is almost never what you want, as it's the most generic. Instead, consider annotating it with text if it's not code, or using #s to get a working example that only shows the part you care about.

Furthermore, another part of the documentation lists ignore as useful for:

Sometimes a few specific tests can be very time-consuming to execute, so you might want to exclude them during most runs of cargo test. Rather than listing as arguments all tests you do want to run, you can instead annotate the time-consuming tests using the ignore attribute to exclude them

Managed to convert most of the examples except of the 2 macros in rsvg_internals/src/property_macros.rs, which require making a whole chain of private macros public for little added value.