docs: add doctests to every function and a Documenter site

[wardjm]

Adds runnable # Examples to every documented function — 223 jldoctest blocks — plus docstrings for the 28 exports that had none (degree, cdc_components, wrap, the accessors, the dg_* factories), and a Documenter site to render and check them.

The examples assert behaviour rather than illustrate it: the circle's Laplacian spectrum (0, 1, 1, 3.78, …), adjointness of δ against d, antisymmetry of the wedge, ♯∘♭ = id, and each weak-form builder reproducing exactly the matrix its operator carries. They run under Pkg.test() (test/doctests.jl) and in the docs build, sharing the preamble in docs/doctest_setup.jl (dg = 60-point circle, dg3 = 200-point sphere, f = cos θ).

What writing them turned up

Three things that only surfaced because the examples were executed — twice, in different environments:

• A function's individual coefficients are not reproducible when it straddles a degenerate eigenspace. λ=1 has multiplicity two on the circle, so the eigensolver returns an arbitrary orthonormal pair spanning that plane: c[2], c[3] came out 0.6144, 0.35 in one environment and 0.5898, 0.3901 in another. Only the Parseval norm is fixed. Noted on _from_pointwise_basis — any assertion on a single such coefficient is unsound.
• betti_spectrum's harmonic eigenvalue magnitude is machine-dependent. The w = 1e10 penalty multiplies the float noise along with the energy, so values[1] roams 1e-5–1e-3 between BLAS versions. The gap that gives the Betti number does not move. BettiSpectrum now warns: count the gap, never a threshold.
• laplacian(dg, 0) and codifferential ∘ d are not the same matrix. Composing passes through the Gram pseudo-inverse; the dedicated weak builder does not. They agree to ~2.5e-6 on resolved modes and differ by ~17% on the truncation tail (same for div ∘ grad vs -Δ). Documented in Conventions and on LinearOperator.

Also

• DiffusionGeometry, MarkovTriple, ImmersedMarkovTriple and GammaCache gain compact show methods — printing a geometry previously dumped 123 KB of eigenbasis to the terminal.
• docs/plotting.md and docs/upstream-bugs.md move under docs/src/ so they are part of the built site; the three references to them are updated.
• CI gains a docs job that builds the site (and so runs the doctests) with checkdocs=:exports, which fails the build on any undocumented export.

Verification

Full suite green: 2473 passed, 0 failed (Pkg.test(), doctests included). julia --project=docs docs/make.jl builds clean with no doctest failures and no missing-docs errors.