docs: provide screenshots and content for tutorial 3 (position compare) and tutorial 4 (snake scan)

[coretl]

Target pages

• tutorials/tutorial3_position_compare
• tutorials/tutorial4_snake_scan

Both stubs contain fewer than 60 words each and are currently near-empty
placeholders. They need the same depth as tutorials 1 and 2 (~600–1200 words
plus annotated screenshots).

Category: human-capture

These pages can only be completed once a person with hands-on PandA access
supplies annotated screenshots and step-by-step bullets. A doc engineer will
then expand them into finished MyST pages.

What a human must do (~4–6 hours total)

Tutorial 3 — Position compare (tutorial3_position_compare)

On a PandA connected to Web Control:

1. Configure a PCOMP block so that a position input is compared against a
   threshold and generates trigger pulses on the output.
2. Take labelled screenshots at each step (block palette, attribute panel, link
   arrows, waveform output if available).
3. Write numbered steps (goal → prerequisites → procedure → expected output) in
   the same voice as tutorials 1 and 2.
4. Record exact block names, field names, and representative parameter values.

Tutorial 4 — Snake scan (tutorial4_snake_scan)

1. Build a 2D snake scan using a SEQ block and its table editor.
2. Take labelled screenshots of the SEQ table configuration and the resulting
   trigger output.
3. Write numbered steps at the same depth as tutorial 2.

Deposit screenshots under docs/images/ in the docs branch and post the
bullets as a comment on this issue (or open a PR).

Source stubs

• docs/_legacy_rst/tutorials/tutorial3_position_compare.rst — 41 words (stub)
• docs/_legacy_rst/tutorials/tutorial4_snake_scan.rst — 52 words (stub)