fix(docs): reformat Prompt and Theme section and fix MDX code-block bug

Verified every claim in the Prompt and Theme section against the actual
source and corrected several inaccuracies: the Starship wrapper's missing
C3 gate, the fallback prompt's vi-mode states and segment order, the FZF
theme's real location (conf.d/theme.fish, not integrations/fzf.fish) and
color set, and the Catppuccin theme-switch example using the wrong
fish_config subcommand (choose, not save). The right-prompt Docker-context
example was rewritten to show that it's independent of exit status.

While reformatting, found that any block build-manual.py couldn't classify
as shell/table/tree fell back to plain 4-space markdown indentation, which
silently renders as squashed, unreadable paragraph text on any page that
also contains an <Aside> or <FileTree> — MDX has no indented-code-block
syntax, unlike plain Markdown. This affected 07-customization.mdx plus four
other pages. Fixed the fallback to emit a fenced ```text block instead,
since fences work in both MDX and plain Markdown; this also gives every
affected block Starlight's normal code-block styling instead of a bare grey
slab.

docs/fish-config.md is intentionally left stale here — CI regenerates and
auto-commits it from docs/manual/** on push to main.

docs/build-manual.py

@@ -189,7 +189,7 @@ 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
+    fish_default_key_bindings fish_vi_key_bindings fish_config
     """.split()
 )
 
@@ -394,7 +394,13 @@ def _render_para(para: list[str], entry_name: str | None, deeper: bool) -> str:
     table = _as_ruled_table(para) or _as_table(para) or _as_file_tree(para)
     if table is not None:
         return table
-    return "\n".join(INDENT + line for line in para)
+    # MDX (used for any page that also carries an <Aside> or <FileTree>)
+    # has no indented-code-block syntax — a plain 4-space-indented block
+    # silently renders as flowed paragraph text there, collapsing every
+    # line break. A fenced block works in both MDX and plain Markdown, so
+    # it's the only fallback that's safe regardless of which one a given
+    # page ends up promoted to.
+    return "```text\n" + "\n".join(para) + "\n```"
 
 
 def _prettify_block(block: list[str], entry_name: str | None) -> str: