docs: refresh README first fold, reconcile stale benchmark numbers, unify the one-liner

README first fold:
- banner 800px -> 520px
- badges condensed from two paragraphs into one row (Trendshift kept, inline)
- lead with Introduction instead of benchmarks
- new How it works section explaining the add/search loop
- benchmarks moved below the intro and retitled from the date-stamped
  New Memory Algorithm (April 2026)
- dropped Research Highlights, which restated the benchmark table verbatim

Benchmarks: the README was already correct at 92.5 / 94.4, matching
mem0.ai/research and docs/core-concepts/memory-evaluation.mdx. The stale
copies were in the migration guides, which quote the numbers as a live
reason to upgrade. Updated both to 92.5 / 94.4 (+21 / +27).
docs/changelog/highlights.mdx keeps 91.6 / 93.4 inside its dated
2026-04-14 entry, which records what was announced at the time.

One-liner: standardized on 'the memory layer for AI agents', already the
canonical form in cli-spec.json, cli/python/pyproject.toml and the CLI
specification. Swept the remaining variants.

docs/docs.json

@@ -1,7 +1,7 @@
 {
   "$schema": "https://mintlify.com/docs.json",
   "name": "Mem0",
-  "description": "Mem0 is a self-improving memory layer for LLM applications, enabling personalized AI experiences that save costs and delight users.",
+  "description": "Mem0 is the memory layer for AI agents, giving them persistent, personalized context across sessions.",
   "theme": "aspen",
   "colors": {
     "primary": "#8F74E0",