docs: address review comments on the performance blog post

- fix wording: "are new", "for each input source", "the strings", "the last zero to seven bytes", "by" Danila
  Kutenin, "the result of that query"
- introduce `json_document` where it first gets a detail, say that the `-mbranches` flag is not used, explain
  why the position and token bookkeeping overloads are empty, and say that a step's percentage is relative to
  the time before the step and that a median of 4 is the mean of the two middle values
- write percentages without a space before `%` in the post and in the price figure (regenerated)

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

docs/mkdocs/scripts/figures/perf_blog.py

@@ -511,7 +511,7 @@ def price():
     lo, hi = -50, 70
     X = lambda pct: L + (pct - lo) / (hi - lo) * (R - L)
     pct = lambda new, old: round((new / old - 1) * 100)
-    lab = lambda p: (MINUS + "%d %%" % -p) if p < 0 else "+%d %%" % p
+    lab = lambda p: (MINUS + "%d%%" % -p) if p < 0 else "+%d%%" % p
     bh, pitch, gap = 11, 14, 10
     b, desc = [], []
     # the key above the plot