docs: archive node shapes and configuration for 2.0

[svlachakis]

Documents the archive functionality shipping in 2.0 (NethermindEth/nethermind#12943), including the log and block query behavior below the pruning boundary from NethermindEth/nethermind#13036.

What it adds

• New page fundamentals/archive-nodes.md: the three archive shapes (full, windowed, address-slice), how FlatDb.History* (historical state) and History.* (historical blocks and receipts) compose, a per-shape configuration table, fail-closed query semantics below the window, the receiptless variant (Receipt.DeriveFromState), and operational notes (fresh flat history requirement, slice depth validation, pruner pacing, every-block verification).
• fundamentals/history-pruning.md: short additions - sliced addresses keep receipts and bodies beyond the rolling window, rolling pruning publishes the boundary first and reclaims space in the background, and a pointer that History.Pruning covers blocks and receipts only while historical state has its own retention.
• Sidebar renumbering in performance-tuning.md and private-networks.md to make room for the new page after History pruning.

Notes

• Option names and behavior are verified against the release/2.0.0-rc branch, not the PR description; the query behavior below the pruning boundary comes from Log index backfill through sliced receipt islands nethermind#13036, which lands in the release. Defaults are not duplicated into the pages; they stay in the generated options reference.
• The build passes; it warns about the FlatDb.History* and Receipt.DeriveFromState anchors in configuration.md that will exist once the options reference is regenerated for 2.0 (same category of warning as the existing #sync-fastsynccatchupheightdelta and #sync-nonvalidatornode ones).