docs: add the blog post "How 3.13.0 got faster"

A post for the documentation (Home > Blog) on how 3.13.0 became faster:
eight themes with the pull requests behind them, the ideas taken from other
libraries, what did not work, how the fast paths are tested, and the
compile-time cost. Every number comes from a run on 2026-10-27 (3.12.0 against
develop plus the open non-draft PRs, Apple M1 Max) or from the linked PR.

- docs/mkdocs/docs/home/blog/how_3_13_0_got_faster.md: the post.
- docs/mkdocs/scripts/figures/perf_blog.py: draws its 12 figures (overview,
  eight timeline panels, the SWAR test, the cliffs, json_view throughput) as
  inline SVGs in the nj-charts style; data/perf_timeline.csv holds the
  timeline.
- tests/benchmarks/history: history.cpp and history.py time one fixed program
  against the header of every commit that changed it, interleaved, and write
  one CSV row per commit with its PR. The timeline of the post comes from it.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/benchmarks/README.md

@@ -138,6 +138,9 @@ the `View*` benchmarks (in `src/benchmarks_view.cpp`) are only built if the dire
 Only benchmarks present in both result files are compared, so for older releases, either filter the benchmarks or
 build that release's own `tests/benchmarks` against its own header.
 
+To find out *which commit* changed a number, [`history/`](history/README.md) times one fixed program against the
+header of every commit that changed it.
+
 ## Getting stable numbers
 
 - Build and run both versions on the same machine with the same compiler and Google Benchmark, one right after the