docs: Update README and gem description to match new positioning

[jamessewell]

Updates the README to match the new positioning from paradedb/paradedb#5483:

• New tagline ("Search without a second system.") and one-Postgres sub-line
• New large light/dark logo variants
• Intro now links "ParadeDB / pg_search" as a single link
• Updates the gem description to the new positioning

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.40%. Comparing base (457de24) to head (2324d10).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #120   +/-   ##
=======================================
  Coverage   90.40%   90.40%           
=======================================
  Files          20       20           
  Lines        2417     2417           
  Branches      776      776           
=======================================
  Hits         2185     2185           
  Misses        190      190           
  Partials       42       42           

Flag	Coverage Δ
rails-paradedb	90.40% <ø> (ø)
rb3.2	90.68% <ø> (ø)
rb3.3	90.68% <ø> (ø)
rb3.4	90.40% <ø> (ø)
rb4.0	81.91% <ø> (ø)
rl7.2	90.40% <ø> (ø)
rl8.1	81.91% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[philippemnoel]

awesome stuff!