docs(bbr3): fix the markdownlint violation and retire its baseline entry

The previous commit added lines above the fallback section, which moved a
pre-existing MD036 (emphasis used instead of a heading) from line 87 to
line 92. The ratchet keys violations by path and line, so it read that as
one new violation plus one stale baseline entry, and the document gate went
red.

Fix the violation instead of renumbering the baseline: option (1) is now
written like options (2) and (3), with the bold label and the explanation
outside it, so the line is no longer a paragraph consisting only of
emphasis. The baseline entry for it is removed, which tightens the ratchet
to 153 known violations.

Verified with the gate itself: npm run markdown-lint followed by
scripts/check-markdownlint-baseline.sh reports 153 pre-existing and 0 new.

Author: LostAttractor

.markdownlint-baseline.txt

@@ -158,7 +158,6 @@ docs/zh/README.md:115:15 MD009/no-trailing-spaces Trailing spaces [Expected: 0 o
 docs/zh/README.md:273 MD022/blanks-around-headings/blanks-around-headers Headings should be surrounded by blank lines [Expected: 1; Actual: 0; Below] [Context: "## PPPoE"]
 docs/zh/README.md:76 MD028/no-blanks-blockquote Blank line inside blockquote
 docs/zh/README.md:77:36 enhanced-proper-names Proper names should have the correct capitalization [Expected: Linux; Actual: linux]
-docs/zh/bbr3-experimental-testing.md:87 MD036/no-emphasis-as-heading/no-emphasis-as-header Emphasis used instead of a heading [Context: "① 链接级（立即生效，无需重编译，首选）"]
 docs/zh/configuration/dns.md:5 MD025/single-title/single-h1 Multiple top-level headings in the same document [Context: "# Schema"]
 docs/zh/tutorials/run-on-alpine.md:3:8 MD009/no-trailing-spaces Trailing spaces [Expected: 0 or 2; Actual: 1]
 docs/zh/tutorials/run-on-alpine.md:4 MD032/blanks-around-lists Lists should be surrounded by blank lines [Context: "1. Alpine Linux 3.18 或更新版本已完全支..."]

docs/zh/bbr3-experimental-testing.md

@@ -89,7 +89,7 @@ level=debug msg="installing experimental bbr3 congestion controller" cc=bbr3 hin
 
 ## 4. 回退（三级，任选）
 
-**① 链接级（立即生效，无需重编译，首选）**
+**① 链接级**（立即生效，无需重编译，首选）
 
 ```text
 # 去掉 cc_override，回到服务端回显