Document v2 fuel-moisture, irradiance, inventory-canopy, and point-cloud data grids

[amarcozzi]

Adds how-to documentation for six recently shipped SDK features. Docs-only — no code under `fastfuels_sdk/` is touched, and the auto-generated Reference is left alone.

`docs/v2/guides/creating-grids.md`

• Solar irradiance grids (LeafLux) — new top-level section for `create_irradiance_grid_from_leaflux`. Covers the canopy band (no terrain) and the surface band's requirement for a terrain grid on the source's exact horizontal lattice with an `elevation` band, built via `align_to=`, with a cross-link to Align grids to each other.
• Fuel moisture grids (Fosberg) — new top-level section for `create_dead_fuel_moisture_grid_from_fosberg`, written from the verified end-to-end topo + LAD + irradiance flow, emphasizing the shared-lattice requirement.
• Canopy fuels from a tree inventory — new subsection under Canopy grids for `create_canopy_fuel_grid_from_inventory` (`cbd`/`cbh`/`chm`/`cc`/`cfl`), pointing to the Reference for the full method surface.
• Seasonal FBFM40 — the FBFM40 subsection now documents `version` (`2024`/`2025`) and `grid.represented_year`.
• CHM aggregation + spike filter — expanded the point-cloud CHM example with `aggregation`/`percentile` and `spike_filter`.

`docs/v2/guides/point-clouds.md`

• Read point cloud data — new section for `PointCloud.metadata()`, `to_numpy()`, and `to_dataframe()`, mirroring the verified data-out test flow.

All examples mirror the verified live/unit test flows in `tests/v2/test_grids.py` and `tests/v2/test_point_clouds.py`. No new nav entries needed (all are subsections of existing pages).

Merge order

The Fuel moisture section documents `create_dead_fuel_moisture_grid_from_fosberg`, which lands via #197 / PR #207. This docs PR should merge after #207.