feat(marketing): make /sources a hero page carrying source provenance and tier weights

## The insight
Alessandra's complaint about Dataminr was, verbatim: *"wanting to know where a
piece of information came from, and you couldn't... there was lots of mystery
around the sourcing."*

We have the answer to that, publicly and free, and it is buried:
- A named, filterable **534-provider catalogue with direct URLs** at `/sources`
- **Quantified tier weights** at `/docs/algorithms`: "Tier 1 `+35`, Tier 2
`+20`, Tier 3 `+8`", source-count boost `min(sourceCount,6)*12`, and the gate
that "Tier 3+ sources require a non-keyword classification to fire an alert;
Tier 1-2 sources bypass this gate"
- 38 OpenAPI specs and a no-key sandbox

## The problem
It is scattered across `/sources`, `/docs/source-attribution`, `/docs/algorithms`
and `/docs/decision-signal-provenance`, and **nowhere is it framed as the answer
to that complaint.** It reads as reference documentation.

## Fix
Make `/sources` a hero page, not a footer link. Pull tier weights and the
provenance model onto it. Lead with the claim, not the inventory.

## Acceptance
- [ ] `/sources` promoted in primary navigation
- [ ] Framed as an answer to "where did this come from?"
- [ ] Linked from the landing page above the fold

Effort: M (~2d).

Start at the /sources route and inspect the existing /docs/algorithms, /docs/source-attribution, and /docs/decision-signal-provenance pages, then trace the primary navigation and landing-page links. Done means /sources is promoted in primary navigation, frames provenance as the answer to “where did this come from?”, and is linked above the fold on the landing page.

Written by the indexing model from the issue text.