Refactor "PDB distributions" section of README

[MattKotsenas]

Refactor the "PDB distributions" section of the README so that:

• There's less bouncing back and forth between text (specifically shrinking the distance between introducing a .snupkg and talking about the limitations)
• Add text offering embedded as a solution when .snupkgs don't work. I think for Source Link users this should be preferred over including .pdbs in the package because that requires consumers to know and set CopyDebugSymbolFilesFromPackages
• Use GitHub markdown tags for asides or secondary information

[MattKotsenas]

I originally wrote this change as part of #1270, but separated it into its own PR since this one is a bit more nuanced and contains some opinions instead of just facts.

More specifically, the only downside I see for advocating for embedded pdbs over separate files included in the main package is it's easier to strip separate files for people that really care about size. I'm not convinced that requiring every consumer to opt-in via CopyDebugSymbolFilesFromPackages is a good tradeoff though, and they should instead either publish on a platform that supports snupkg or use another solution like symbol servers.

[MattKotsenas]

@tmat, would you mind taking a look at this when you have a second? If you have any questions or concerns, please let me know. Thanks!

[tmat]

I would suggest including portable PDBs in the nupkg first and embedding PDBs into the assembly as an alternative. The latter increases size of the assembly, which may potentially be an issue in some cases.