add markdown docs for archival snapshots

Author: lemmih

docs/docs/users/guides/advanced/archival_snapshots.md

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+---
+title: Archival Snapshots
+sidebar_position: 2
+---
+
+# Archival Snapshots
+
+ChainSafe hosts two kinds of snapshots: hourly snapshots (guaranteed to be no
+more than a few hours old) and archival snapshots (similar to the regular
+snapshots, but with less duplicate data). Archival snapshots come as 'lite'
+snapshots, which include the entire block header history back to genesis, and
+'diff' snapshots, which only contain the new data since the previous diff
+snapshot.
+
+Archival snapshots are publicly available here:
+
+- Mainnet lites: https://forest-archive.chainsafe.dev/list/mainnet/lite
+- Mainnet diffs: https://forest-archive.chainsafe.dev/list/mainnet/diff
+- Calibnet lites: https://forest-archive.chainsafe.dev/list/calibnet/lite
+- Calibnet diffs: https://forest-archive.chainsafe.dev/list/calibnet/diff
+
+## Merging snapshots
+
+Since 'diff' snapshots only contain the new data since the previous diff
+snapshot, they need to be merged with the previous 'lite' snapshot to form a
+complete snapshot. This can be done with the `forest-tool archive merge`
+command.
+
+```shell
+forest-tool archive merge --output-file <output-file> <lite-snapshot> <diff-snapshots>
+```
+
+As an example, to get a snapshot that covers epoch 30_000 to epoch 36_000, you
+merge `forest_snapshot_mainnet_2020-09-04_height_30000.forest.car.zst` with
+`forest_diff_mainnet_2020-09-04_height_30000+3000.forest.car.zst` and
+`forest_diff_mainnet_2020-09-05_height_33000+3000.forest.car.zst`.
+
+## Generating archival snapshots
+
+New archival snapshots can be generated either manually with `forest-tool
+archive export` or automatically with `forest-tool archive sync-bucket`. Both
+commands require a large snapshot file as input.
+
+To generate archival snapshots manually, use these settings:
+
+- one lite snapshot every 30_000 epochs,
+- one diff snapshot every 3_000 epochs,
+- a depth of 900 epochs for the diff snapshots,
+- a depth of 900 for the lite snapshots.
+
+Manual generation of archival snapshots should be a last resort. The
+`forest-tool archive sync-bucket` command is recommended for generating
+archival snapshots.