chore: revise contribution guide

Updates the contribution guide to have up-to-date instructions to build and test GNOME Podcasts.

Reference: #129 (comment 1846517)

• Adds a first step, which is to call meson setup with profile=development. This generated files expected to compile the project.
• As test.sh has the cargo test… command, just use this everywhere when telling devs to run tests.
• Fix link to Archive.org, otherwise it looks like a local file instead of an HTTP(S) URL.
• Remove the psudo-heading for installing rustfmt. Using bold instead of an actual heading is just a bit messy, and the documentation is clear enough as it is.
• Remove "As of 2019" as a contributing guide should be a living document. (i.e. when that fact changes, the contribution guide should change with it, so dating it gives the reader unnecessary information)