build: fix md parsing for file paths with variables

Updated the python site builder's regex to correctly parse file paths containing shell variables like $__fish_user_dots_path in comment titles. Also added fish_default_key_bindings to the known shell-heads list so those examples are correctly wrapped in a fenced code block rather than falling through to plain text and getting parsed as MDX headings.

docs/build-manual.py

@@ -160,6 +160,7 @@ SHELL_HEADS = frozenset(
     if jobs kitty ls man math mkdir mv nvim npm pacman paru pip pip3 pkg printf
     python python3 rm set shutdown source string sudo switch systemctl test time
     tmux touch trash type wget wezterm while yay zellij zypper
+    fish_default_key_bindings fish_vi_key_bindings
     """.split()
 )
 
@@ -212,7 +213,7 @@ PATH_LINE_RE = re.compile(r"^[~$][\w./{}-]*\.\w+$")
 # A leading "# in local.fish" / "# local.fish" comment names the file an
 # example belongs to; promote it to the fence title instead of leaving it
 # as a literal comment inside the code.
-FILENAME_COMMENT_RE = re.compile(r"^#\s*(?:in\s+)?([\w-]+\.\w+)\s*$")
+FILENAME_COMMENT_RE = re.compile(r"^#\s*(?:in\s+)?([$~\w./-]+\.\w+)\s*$")
 
 CELL_SPLIT = re.compile(r"\s{2,}")