[UG] Fix links and follow style guide re. drop-down

[chalin]

• Edits UG to conform to https://developers.google.com/style/word-list#drop-down

  In most cases, you can omit drop-down from phrases like drop-down list or drop-down menu, and just use list or menu.

• Fixes invalid links: "hash does not exist" errors
• Drops node installation instructions that are way too detailed for the UG, which isn't an Ops manual!

/cc @LisaFC @emckean