docs: correct README and metrics drift

[dknauss]

Summary

Correct the documentation drift identified in the current-state review.

Changes

• Remove RIS from the README import instructions; RIS remains export-only.
• Clarify that the plugin has no citation tables or long-lived settings rather than claiming no database storage.
• Refresh README and WordPress.org footprint figures.
• Add 1.5.0 and 1.5.1 to the recent release highlights.
• Refresh metrics provenance and record the published v1.5.1 release asset size.
• Make metrics re-derivation commands copy-safe in Markdown tables.

Verification

• Compared the branch against current main.
• Confirmed version and release metadata against the v1.5.1 GitHub release.
• Local tests could not be run because the current session's terminal is unavailable; no code behavior was changed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.06%. Comparing base (ce3a862) to head (63fe5e4).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #84   +/-   ##
=======================================
  Coverage   81.06%   81.06%           
=======================================
  Files          43       43           
  Lines        3153     3153           
  Branches      523      523           
=======================================
  Hits         2556     2556           
  Misses        191      191           
  Partials      406      406           

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