docs: cover help popover, +/- nav indicators, render_tool_header

User-facing docs (USER-GUIDE en+es, README en+es):
- New short paragraph under §3.1 GUI noting the in-tool Help button
  on every detail page, what it contains (When to use / Steps /
  Examples / Tip), and that content lives in tools.<id>.help_md.
- One-line note in the README tool tables pointing at the same.
- Mention the sidebar +/- nav indicators replacing Streamlit's
  default Material Symbols chevron.

Developer docs:
- DEVELOPER: new "Tool page header" subsection documenting
  render_tool_header(tool_id), the help_md markdown skeleton, and
  the fallback to help.missing_body when a tool's help is absent.
  Update i18n authoring rules to list help.* keys and the per-tool
  help_md field alongside name/description/page_title/page_caption.
- TECHNICAL: new §10c documenting the sidebar nav indicator swap —
  CSS in _HIDE_CHROME_CSS plus _SWAP_NAV_SECTION_INDICATOR_JS
  injected through the hide_streamlit_chrome() iframe bundle.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/USER-GUIDE.md

@@ -135,6 +135,10 @@ Full numbered support matrix: [REQUIREMENTS.md](REQUIREMENTS.md).
 
 Advanced options are tucked in expander panes. The original file is never modified.
 
+**In-tool Help**: every tool page has a **Help** button right of the title. Click it to open a popover with a compact how-to (When to use · Steps · Examples · Tip). Use it as a refresher mid-task — the popover closes when you click outside, your inputs are untouched.
+
+**Sidebar nav**: the sidebar groups tools into sections (Analysis, Data Cleaners, Transformations, Automations). Each section header shows `+` when collapsed and `−` when expanded — click the header to toggle.
+
 ### 3.2 CLI
 
 ```bash