[#253] clean up INSTALL.md and add instructions to run veilid-server

I saw that #253 (closed) wasn't closed and took the opportunity to:

• do some clean up on INSTALL.md
• add some instructions to run veilid-server, since it was also asked in the MR and "how to run" is the logical follow-up to "how to install"

This MR:

• Cleans up some markdown formatting.
• Makes sections more concise.
• Prunes some instructions.
• Adds a 'run veilid-server' section

Changes that add instructions to run veilid server:

• Changed title to include run
• Added systemctl start ... instructions
• Added heavily caveated instructions to run veilid-server manually

Changes for cleanup:

• Added Install section to group debian and rpm installs.
• Shortened titles Add the repo to a Debian ... and ... Fedora ... to keep it super concise and decoupled from actual steps
• Some formatting best-practices
  • newlines between markdown blocks
  • indent code blocks that belong to bullet points to make sure that they are syntatically grouped (i.e. correct markdown 'DOM' hierarchy)
• sudo:
  • Removed sudo explanations. apt/dnf/yum will not run without, no need to explain.
  • added sudo to RPM instruction code blocks (just those two) for consistency, and also you can't run dnf/yum without root anyway.
• Made generic "Fedora based" instructions. Actually YUM/DNF-based, keeping it simple for now. Also name-dropped common distros for SEO/CTRL+F convenience
• Removed certain Explanation blocks for concision. Some are repeated, and some step titles are already self-explanatory