add sys canisters (#3915)

docs/developer-docs/smart-contracts/advanced-features/system-canisters.mdx

@@ -8,8 +8,6 @@ import { MarkdownChipRow } from "/src/components/Chip/MarkdownChipRow";
 
 <MarkdownChipRow labels={["Intermediate", "Concept"]} />
 
-
-
 System canisters are canisters that provide a necessary function to the ICP network. They are controlled by the NNS and upgraded via NNS proposals. These canisters have static canister IDs that projects can make calls to. One common example of a system canister that is used by projects is the Internet Identity canister.
 
 ## System canisters
@@ -18,7 +16,10 @@ System canisters are canisters that provide a necessary function to the ICP netw
 |-----------------|-------------|
 | [Bitcoin mainnet](/docs/current/developer-docs/multi-chain/bitcoin/overview) | ghsi2-tqaaa-aaaan-aaaca-cai |
 | [Bitcoin testnet](/docs/current/developer-docs/multi-chain/bitcoin/overview) | g4xu7-jiaaa-aaaan-aaaaq-cai |
+| [Cycles ledger](/docs/current/developer-docs/defi/cycles/cycles-ledger) | um5iw-rqaaa-aaaaq-qaaba-cai |
+| [Cycles ledger index](/docs/current/developer-docs/defi/cycles/cycles-ledger) | ul4oc-4iaaa-aaaaq-qaabq-cai |
 | [Cycles minting](/docs/current/developer-docs/gas-cost) | rkp4c-7iaaa-aaaaa-aaaca-cai |
+| [Exchange rate canister](/docs/current/developer-docs/defi/exchange-rate-canister) | uf6dk-hyaaa-aaaaq-qaaaq-cai |
 | [Genesis token](/how-it-works/tokenomics/) | renrk-eyaaa-aaaaa-aaada-cai |
 | [ICP index](/docs/current/developer-docs/defi/tokens/indexes) | qhbym-qaaaa-aaaaa-aaafq-cai |
 | [ICP ledger](/docs/current/developer-docs/defi/overview) | ryjl3-tyaaa-aaaaa-aaaba-cai |