CH02::block explorers: modernize list and add warning

Replace older block explorers with modern alternatives and warn readers
that looking up info on explorers may disclose their interest to third
parties.

ch02.asciidoc

@@ -40,9 +40,9 @@ image::images/mbc2_0201.png["Bitcoin Overview"]
 
 ((("Bitcoin Block Explorer")))((("BlockCypher Explorer")))((("blockchain.info")))((("BitPay Insight")))Popular blockchain explorers include:
 
+* https://blockstream.info/[Blockstream Explorer]
+* https://mempool.space[Mempool.Space]
 * https://live.blockcypher.com[BlockCypher Explorer]
-* https://blockchain.info[blockchain.info]
-* https://insight.bitpay.com[BitPay Insight]
 
 Each of these has a search function that can take a Bitcoin address,
 transaction hash, block number, or block hash and retrieve corresponding
@@ -50,6 +50,17 @@ information from the Bitcoin network. With each transaction or block
 example, we will provide a URL so you can look it up yourself and study
 it in detail.
 
+[WARNING]
+====
+Searching information on a block explorer may disclose to its operator
+that you're interested in that information, allowing them to associate
+it with your IP address, browser fingerprint, past searches, or other
+identifiable information.  If you look up the transactions in this book,
+the operator of the block explorer might guess that you're learning
+about Bitcoin, which shouldn't be a problem.  But if you look up your
+own transactions, the operator may be able to guess how many bitcoins
+you've received, spent, and currently own.
+====
 
 [[bitcoin_e_commerce]]
 ==== Buying from an Online Store