Docs: explain where the data comes from and how sources are arbitrated

[lambda2]

A page for the reader who wants to judge whether to trust a value, without reading any code.

Why a second page on this

Data provenance documents the facts endpoint — it answers where did this value come from for someone holding an API token and reading JSON. It does not answer what a botanist or a data consumer asks first: what are these sources, why does one beat another, and how much of this can I rely on?

This is the editorial companion, and it sits first in the Advanced section so it is read before the field reference.

What it covers

• The sources, what each one actually contributes, and roughly how much of the dataset it covers — including that Baseflor is ~6 400 species of western European flora, not a global layer
• Why Wikipedia never feeds a numeric field: an article quoting a height is quoting a source, and we would rather cite that source
• The four principles behind the ranking — human review over automated import, measurement over assertion, regional flora over global aggregator within its region, nomenclatural authority over computed consensus
• That the ranking is an editorial judgement, stated plainly, including its known limitation: it is global where authority is often field-specific
• What the tricky fields mean, in particular that ecological indicators describe habitat, not tolerance, cover only the temperate European flora, and that 0 is a value there rather than a blank — the exact confusion behind trefle-api#90
• Why a range never becomes an average: a midpoint would be our arithmetic, not anyone's observation
• An honest account of coverage: nomenclature is in good shape, botanical traits are largely empty, and the page says so — "the database is not wrong on these fields; it is empty, which is a different problem and, in our view, a more honest one"
• What a low completeness figure means, so it is not read as unreliability

Verification

Build green, new page generated, both internal links resolve (onBrokenLinks defaults to throw in Docusaurus 3, so a bad link would have failed the build).