docs: finishing chain-indexer-overview-for-operators.md (#12600)

* Followup to PR #12450 for doc updates

This is being used to resolve the unresolved items in #12450 since that PR is unwieldly at this point.

* Incorporated some items and added TODOs based on unresolved items from #12450

* Incorporating more feedback

* Pointing to issue to learn about benefits

* Formatting fixes

* Apply most of the suggestions from @rvagg code review

Co-authored-by: Rod Vagg <rod@vagg.org>

* Incorporating feedback from #12600 (comment)

* Addressing #12600 (comment) and more

* Moved chain-indexer docs to documentation
Renamed
Added ToC

We can move to lotus-docs later

* Update documentation/en/chain-indexer-overview-for-operators.md

Co-authored-by: Rod Vagg <rod@vagg.org>

* Update documentation/en/chain-indexer-overview-for-operators.md

Co-authored-by: Rod Vagg <rod@vagg.org>

* Added upgrade path when importing chain state from a snapshot.

* Typo fixes

* Update documentation/en/chain-indexer-overview-for-operators.md

Co-authored-by: Rod Vagg <rod@vagg.org>

* chore(doc): "regular checks" section for chainindexer docs (#12612)

* Apply suggestions from @rvagg code review

Co-authored-by: Rod Vagg <rod@vagg.org>

* Incorporating @aarshkshah1992 feedback

* Update documentation/en/chain-indexer-overview-for-operators.md

Co-authored-by: Rod Vagg <rod@vagg.org>

---------

Co-authored-by: Rod Vagg <rod@vagg.org>
Co-authored-by: Aarsh Shah <aarshkshah1992@gmail.com>

CHANGELOG.md

@@ -1,15 +1,14 @@
 # Lotus changelog
 
 # UNRELEASED
- (`ChainIndexer`) to index Filecoin chain state such as tipsets, messages, events and ETH transactions for accurate and faster RPC responses. The `ChainIndexer` replaces the existing `MsgIndex`, `EthTxIndex` and `EventIndex` implementations in Lotus, which [suffer from a multitude of known problems](https://github.com/filecoin-project/lotus/issues/12293).  If you are an RPC provider/node operator, please refer to the [ChainIndexer documentation for RPC providers](TODO: URL) for information on how to enable, configure and use the new Indexer.
-Add `EthGetBlockReceipts` RPC method to retrieve transaction receipts for a specified block. This method allows users to obtain Ethereum format receipts of all transactions included in a given tipset as specified by its Ethereum block equivalent. ([filecoin-project/lotus#12478](https://github.com/filecoin-project/lotus/pull/12478))
-
 
 ## ☢️ Upgrade Warnings ☢️
 
 - Minimum go-version been updated to v1.22.7 ([filecoin-project/lotus#12459](https://github.com/filecoin-project/lotus/pull/12459))
 
 ## New features
+- New ChainIndexer subsystem to index Filecoin chain state such as tipsets, messages, events and ETH transactions for accurate and faster RPC responses. The `ChainIndexer` replaces the existing `MsgIndex`, `EthTxHashLookup` and `EventIndex` implementations in Lotus, which [suffer from a multitude of known problems](https://github.com/filecoin-project/lotus/issues/12293).  If you are an RPC provider or a node operator who uses or exposes Ethereum and/or events APIs, please refer to the [ChainIndexer documentation for operators](./documentation/en/chain-indexer-overview-for-operators.md) for information on how to enable, configure and use the new Indexer.  While there is no automated data migration and one can upgrade and downgrade without backups, there are manual steps that need to be taken to backfill data when upgrading to this Lotus version, or downgrading to the previous version without ChainIndexer. Please be aware that that this feature removes some options in the Lotus configuration file, if these have been set, Lotus will report an error when starting. See the documentation for more information.
+- Add `EthGetBlockReceipts` RPC method to retrieve transaction receipts for a specified block. This method allows users to obtain Ethereum format receipts of all transactions included in a given tipset as specified by its Ethereum block equivalent. ([filecoin-project/lotus#12478](https://github.com/filecoin-project/lotus/pull/12478))
 - Update `EthGetBlockByNumber` to return a pointer to ethtypes.EthBlock or nil for null rounds. ([filecoin-project/lotus#12529](https://github.com/filecoin-project/lotus/pull/12529))
 - Reduce size of embedded genesis CAR files by removing WASM actor blocks and compressing with zstd. This reduces the `lotus` binary size by approximately 10 MiB. ([filecoin-project/lotus#12439](https://github.com/filecoin-project/lotus/pull/12439))
 - Add ChainSafe operated Calibration archival node to the bootstrap list ([filecoin-project/lotus#12517](https://github.com/filecoin-project/lotus/pull/12517))