benchmark: add Abseil comparison mode and refresh docs

[sukeya]

Add an Abseil-backed benchmark configuration for ordered containers and
tighten the comparison target to a single platanus node-size when that
mode is enabled.

The benchmark build now supports a dedicated comparison mode controlled
by PLATANUS_BENCHMARK_WITH_ABSL. When enabled, CMake fetches
abseil-cpp 20260107.1 as a benchmark-only dependency and builds
btree_bench against Abseil's btree containers. In this mode the
registered implementations are intentionally restricted to:

• STL
• absl
• platanus(64)

while platanus(128) and all pmr benchmark variants are excluded.

Update benchmark registration in btree_bench.cpp to add AbslSet,
AbslMultiSet, AbslMap, and AbslMultiMap coverage for Insert, Lookup,
Delete, FwdIter, and Merge across int32_t, int64_t, and string test
data. Keep the previous wider registration behavior when the Abseil
comparison mode is disabled.

Teach benchmark/compare_benchmarks.py to recognize Absl* benchmark
names, place "absl" between STL and platanus on the x-axis, and accept
platanus::kAutoSize as "platanus(auto)". Preserve single-file and
left/right comparison modes, and keep per-implementation coloring for
single-series plots.

Fix benchmark/compare_benchmark_stats.py argument parsing by removing
invalid positional required=True usage so the script works reliably in
its two-input comparison mode.

Refresh the benchmark and reference documentation to match the current
code:

• document the Abseil comparison mode and its scope
• document the benchmark build commands and plotting workflow
• update reference signatures/defaults to reflect kAutoSize and
  NodeByteSize
• fix stale wording and minor typos

This keeps the benchmark story aligned with the current implementation
and makes STL vs. absl vs. platanus(64) comparisons reproducible from a
fresh build tree.