ch-channel-graph: add chapter overview

channel-graph.asciidoc

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+Chapter overview:
+  * explains the channel graph, and how it's modified+verified
+
+Relevant questions to answer:
+  * Gossip announcements:
+      * How does a peer announce a new channel to the network?
+      * How do nodes verify a channel announcement? Why should they verify one in the first place?
+      * How does a node control _how_ a payment is routed through its channel?
+      * What knobs exist for a node to set in their channel updates?
+      * How often are channel updates sent?
+      * How does a node update its node in the channel graph? Do we we need to verify this?
+      * How quickly does an update propagate?
+      * What are "zombie" channels? Why do they matter?
+  * Channel graph syncing:
+      * What are the various ways a node can sync the channel graph?
+      * Which is the most efficient?
+      * What is the "gossip query" system?
+      * Does a node need to keep up with all gossip updates? Does this change if they're a routing node or mobile client?
+  * Protocol Extensions via Feature Bits and TLV:
+      * How can the channel graph be upgraded using feature bits and TLV fields?
+      * How does a receiver signal that they can accept MPP/AMP payments?