Update install badges for better readability, faster loading

[jqnatividad]

Currently, the install badges have the full target platform in a small font, e.g.

x86_64-unknown-linux-gnu

since the operating system is already prominent along with an image to represent it, let's just use a subset of the full target string using a bigger, more readable font - e.g.

x86_64-unknown-linux-gnu -> x86_64-gnu
x86_64-unknown-linux-musl -> x86_64-musl
powerpc64le-unknown-linux-gnu -> powerpc64-gnu
aarch64-unknown-linux-gnu -> aarch64-gnu
x86_64-unknown-linux-musl -> x86_64-musl
x86_64-pc-windows-msvc -> x86_64-msvc
x86_64-pc-windows-gnu -> x86_64-gnu
aarch64-pc-windows-msvc -> aarch64-msvc

at the same time, use png instead of the original svg to make the badges load faster.