docs: flat-history retention modes, including SinceBlock

[svlachakis]

Follows NethermindEth/nethermind#13162, which makes the windowed-archive shape explicit instead of implied by a block count, and adds a fixed-floor mode:

FlatDb.HistoryRetention           = None (default) | Rolling | SinceBlock
FlatDb.HistoryRetentionBlocks     = the window size, required when Rolling
FlatDb.HistoryRetentionSinceBlock = the first block to keep, required when SinceBlock

Updates the archive-nodes page: the configuration table gains the mode and since-block rows, a since-block shape bullet explains what it keeps and pairs it with UseAncientBarriers for blocks and receipts, and the fresh-history warning names both bounded modes.

Docs-only; merge alongside or after the code change.