pageserver: improve readability of shard.rs (#7330)

No functional changes, this is a comments/naming PR.

While merging sharding changes, some cleanup of the shard.rs types was
deferred.

In this PR:
- Rename `is_zero` to `is_shard_zero` to make clear that this method
doesn't literally mean that the entire object is zeros, just that it
refers to the 0th shard in a tenant.
- Pull definitions of types to the top of shard.rs and add a big comment
giving an overview of which type is for what.

Closes: https://github.com/neondatabase/neon/issues/6072

storage_controller/src/service.rs

@@ -2744,7 +2744,7 @@ impl Service {
         let mut describe_shards = Vec::new();
 
         for shard in shards {
-            if shard.tenant_shard_id.is_zero() {
+            if shard.tenant_shard_id.is_shard_zero() {
                 shard_zero = Some(shard);
             }
 
@@ -4084,7 +4084,7 @@ impl Service {
 
         let mut reconciles_spawned = 0;
         for (tenant_shard_id, shard) in tenants.iter_mut() {
-            if tenant_shard_id.is_zero() {
+            if tenant_shard_id.is_shard_zero() {
                 schedule_context = ScheduleContext::default();
             }
 
@@ -4134,7 +4134,7 @@ impl Service {
         let mut work = Vec::new();
 
         for (tenant_shard_id, shard) in tenants.iter() {
-            if tenant_shard_id.is_zero() {
+            if tenant_shard_id.is_shard_zero() {
                 // Reset accumulators on the first shard in a tenant
                 schedule_context = ScheduleContext::default();
                 tenant_shards.clear();