docs: convert the software page to the house style

[brickbots]

Full simplified technical English pass over docs/source/software.rst, using the docs skill's seven rules and approved-term table. Part of the manual-wide conversion, one PR per file.

Measured change

  PASS  docs/source/software.rst
     em-dash 0->0 | semicolon 1->0 | banned 6->1 | words 936->914 (-2%)
     terms: choose 5->1, the unit 1->0
OVERALL: PASS

What the converting agent reported

EM_DASHES: 0 -> 0
SEMICOLONS: 1 -> 0
TERMS: "deep sky catalog images" -> "deep-sky catalog images" (the manual's one
  real four-noun stack, rule 6); choose->select x3 (literal imager buttons
  'Choose OS'/'Select Storage' left verbatim); "the unit"->"your PiFinder";
  "power it up"->"turn it on" x2; boot->start x3; reboot->restart;
  wifi/Wifi/WIFI -> WiFi x7 (dominant form elsewhere in the manual);
  pre-built->prebuilt; "sd card"->"SD card"; burn->write (matches the
  imager's own Write button)
STRUCTURAL: no reorders. 5 run-ons split. Passives made active ("SSH is enabled
  on this image" -> "This image enables SSH"). Flashing steps made imperative.
  Auxiliary chains cut ("will probably take" -> "takes"). 936 -> 914 words.
LEFT_ALONE: all 5 headings and underlines (:ref: targets used by sd_card.rst,
  dev_guide.rst and user_guide.rst); every command, URL, code block and the
  blockquote around the wget line; step order in both procedures; contractions,
  which are the manual's register and not forbidden by STE.
FACT_CONCERNS:
  1. Likely mis-indentation, left as found: the five SPI/I2C bullets are nested
     under the "sudo apt upgrade" bullet, so they render as sub-steps of the
     upgrade rather than a separate setup step. Fixing it changes structure.
  2. Three images escape docs/source/ into the repo-root images/ tree, unlike
     the four images/software/... paths on the same page. All exist, build is
     clean, but it breaks the per-page images/<page>/ convention. Pre-rev4 shots.
  3. "Build From Scratch" still specifies 64-bit Pi OS Legacy (Bullseye). If the
     supported base OS has moved on, this section is stale.

Content changes (added after review)

Two follow-up commits, neither of them style:

• The SPI/I2C steps were mis-nested. "Enable SPI / I2C" and its four
  raspi-config steps rendered as sub-items of sudo apt upgrade, two levels
  below where they belong, which made enabling the buses look like part of
  updating packages. They are now a sibling of "Update all packages", with the
  raspi-config steps nested under them. Confirmed in the built HTML: the item
  now sits at the same list depth as "SSH into the PiFinder". No wording
  changed.
• The last stray "reboot" is now "boot". The page normalised
  reboot -> restart everywhere else, leaving one "start on reboot". "restart" is
  the wrong replacement: the service starts on every boot, not only after a
  restart. Under the settled turn on vs boot rule (docs skill: split "turn on" from "boot" in the term table #599), "boot" is the
  approved word for the machine's own startup sequence.

Safety

Every other change in this PR is style only. No numbers, procedures or step order were changed.

Verified mechanically against origin/main that every heading (text and underline character), every :ref:, :doc:, image path, substitution, include:: and external URL is unchanged. Headings matter because autosectionlabel turns each one into a cross-reference target that other pages depend on, and a rename would break them silently.

Sphinx builds clean under -n (nitpicky).

Any FACT_CONCERNS above that are not listed under Content changes were deliberately not fixed. They are reported for a maintainer decision, since changing them would be a content edit rather than a style one.

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