docs: reading.md lists masks among the sections in their one order

[FabianHofmann]

Prompt: "File (U1), (F11), (F12) and (I3) as issues, or fix (F12) and (I3) in one small PR." — answered "3"

Note

The following content was generated by AI.

docs/reference/reading.md gave the canonical section order without masks, which #821 added. The list now matches Spec's field order and SECTIONS in tests/test_canonical.py.

Method, gate output, alternatives

Check. The fields of Spec in src/mathspec/spec.py and SECTIONS in tests/test_canonical.py give one order: version, description, given, dimensions, relations, parameters, variables, constraints, objective, expressions, masks, macros, piecewise, sos, assumptions. reading.md had every one of these but masks. given was already third, after version and description, as #818 puts it.

Sweep. I searched docs/, README.md and src/ for other lists of the sections. docs/reference/language/file.md lists the thirteen declaration keys with masks in its place. No other page gives the order.

Gates. pixi run lint and pixi run ci pass in the worktree (2841 passed, 1 skipped; docs-build and compile-tex pass). The CHANGELOG line is a second commit, pushed after the PR number existed; only pixi run lint ran on that commit.