enh(README) New screenshots, setup/troubleshooting instructions

[joshtrichards]

• Adds/updates screenshots
• Adds basic Installation instructions
• Adds basic Configuration instructions
  • Add red Caution warning about importance and inability to recover mnemonic/encryption passphrase
• Adds basic Usage instructions
• Adds basic Troubleshooting notes

Still missing (but also unnecessary for this PR):

• iOS client screenshot (not critical since this is the server component but noting since I didn't intentionally exclude it and in case someone wishes to add one as a follow-on)

Possible future follow-ups:

• Links to the client repositories (for looking up or reporting issues, for checking release notes for fixes/enhancements)
• Tips for commonly reported problems in this or the client repositories (FAQs or, in some cases, "known issues")
• Tips/acknowledgements of some of the more commonly reported historical issues that are no longer considered "known issues" since they are believed to be resolved
• Fix Security Policy link (currently goes nowhere for some reason)
• Usage/Troubleshooting instructions:
  • Changing mnemonics
  • Resetting mnemonics
• Add info how to build the app #340

Note: Not necessarily all of the above belong here, but noting for future ideas.

[github-actions]

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