docs(user-guide): say which key confirms, and that SQUARE does not

[brickbots]

A new rev4 owner spent a first night unable to confirm a shutdown and kept landing in the
help menu instead. The canonical key list in user_guide.rst ("The Menu System") stopped
at scroll and select, so nothing told him that RIGHT is the confirm key on a
confirmation screen, or that SQUARE opens the Quick Menu rather than confirming. The
Quick Start's alignment steps reinforced the wrong habit by presenting SQUARE as the key
that "completes" a step.

What changed

docs/source/user_guide.rst

• Expanded the existing key list under "The Menu System" with two bullets: RIGHT is also
  the confirm key (on a confirmation screen it selects the highlighted option, LEFT goes
  back), and SQUARE does not confirm (press and hold for the Quick Menu, a short press
  does whatever the current screen uses it for).
• Extended the joystick sentence that follows, so it now says that holding the joystick in
  opens the Quick Menu, and that RIGHT is what confirms.
• "Shutdown": added a :ref: back to user_guide:the menu system, plus a line saying
  SQUARE does not confirm on that screen.
• "Power button and shutdown" and "Shutdown": the confirming power-button press is a press
  and hold, not a short press (see below), and both passages now name RIGHT as the
  alternative.

docs/source/quick_start.rst

• Mirrored both new bullets into the duplicate key list under "Using the PiFinder", keeping
  the two keep-in-sync comments accurate.
• "Alignment" and "Daytime alignment": reframed SQUARE as the screen's own start/save
  action rather than a generic confirm. Step 3 now says "start the alignment" and step 5
  "save the alignment", each naming the hint the screen actually draws (START ALIGN, SAVE),
  followed by one line stating that SQUARE is not a general confirm key. The procedures
  are unchanged in substance.
• "Shutting down the PiFinder": names RIGHT, and corrects the confirming press.

No new screenshots. The images in these sections are already at the rev4 352x352 size.

Key behaviours verified in code

Claim	Source
Joystick centre press sends SQUARE; held, it sends LNG_SQUARE	python/PiFinder/keypad.py — KEYMAP/LONG_KEYMAP position (4,4)
Press and hold SQUARE opens the Quick Menu; a short SQUARE is screen-specific	ui/menu_manager.py key_long_square() vs key_square(); ui/base.py key_square() defaults to cycle_display_mode(), which is a no-op when _display_mode_list is [None]
The shutdown screen is a Confirm/Cancel menu where RIGHT selects and LEFT goes back	ui/menu_structure.py label shutdown; ui/text_menu.py key_right(); ui/menu_manager.py key_left()
A second PWR press on that screen confirms	ui/text_menu.py key_power() calls key_right() when the menu label is shutdown; ui/base.py key_power() jumps to the shutdown label otherwise
HELP is the UP option of the Quick Menu, which is why the reader kept landing there	ui/marking_menus.py — MarkingMenu.up defaults to HELP; ui/text_menu.py gives every text menu a marking menu
SQUARE on the align screens starts, then saves, the alignment	ui/align.py key_square() and its START ALIGN / SAVE / 0 CANCEL hints; ui/align_daytime.py key_square() -> _save_alignment()

Where the issue and the code disagree

The issue says "a second short press of PWR on the shutdown confirm". There is no
short-press path for the power button at all. keyboard_pi.run_keyboard() emits POWER_BTN
only once the GPIO has read low for more than one second, and latches until release, so
every power-button press is a press and hold of about a second, the confirming one included.
The manual said "Press the power button again", which understates the hold and is a plausible
part of why the customer could not confirm. Both shutdown passages now say "press and hold
the power button again".

Two smaller corrections to the issue's wording, both reflected in the text:

• SQUARE does not open the Quick Menu on its own. Press and hold does. A short press is
  handed to the current screen, and on a menu screen it does nothing visible.
• The issue asks for the shutdown section to name the confirm key. user_guide.rst
  "Shutdown" already named RIGHT, so that part needed only the cross-reference plus the
  press-and-hold fix.

Verification

python -m sphinx -b html -E -n -q source /tmp/sphinx_647 prints nothing. No em-dashes or
semicolons in the added lines.

Closes #647

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