[Fixes bevyengine#6059] Entity's “ID” should be named “index” ins…

…tead (bevyengine#6107)

# Objective

Fixes bevyengine#6059, changing all incorrect occurrences of ``id`` in the ``entity`` module to ``index``:

* struct level documentation,
* ``id`` struct field,
* ``id`` method and its documentation.

## Solution

Renaming and verifying using CI. 


Co-authored-by: Edvin Kjell <[email protected]>

crates/bevy_ecs/src/bundle.rs

@@ -543,10 +543,10 @@ impl<'a, 'b> BundleInserter<'a, 'b> {
  InsertBundleResult::NewArchetypeSameTable { new_archetype } => {
  let result = self.archetype.swap_remove(location.index);
  if let Some(swapped_entity) = result.swapped_entity {
- self.entities.meta[swapped_entity.id as usize].location = location;
+ self.entities.meta[swapped_entity.index as usize].location = location;
  }
  let new_location = new_archetype.allocate(entity, result.table_row);
- self.entities.meta[entity.id as usize].location = new_location;
+ self.entities.meta[entity.index as usize].location = new_location;
 
  // PERF: this could be looked up during Inserter construction and stored (but borrowing makes this nasty)
  let add_bundle = self
@@ -571,15 +571,15 @@ impl<'a, 'b> BundleInserter<'a, 'b> {
  } => {
  let result = self.archetype.swap_remove(location.index);
  if let Some(swapped_entity) = result.swapped_entity {
- self.entities.meta[swapped_entity.id as usize].location = location;
+ self.entities.meta[swapped_entity.index as usize].location = location;
  }
  // PERF: store "non bundle" components in edge, then just move those to avoid
  // redundant copies
  let move_result = self
  .table
  .move_to_superset_unchecked(result.table_row, new_table);
  let new_location = new_archetype.allocate(entity, move_result.new_row);
- self.entities.meta[entity.id as usize].location = new_location;
+ self.entities.meta[entity.index as usize].location = new_location;
 
  // if an entity was moved into this entity's table spot, update its table row
  if let Some(swapped_entity) = move_result.swapped_entity {
@@ -655,7 +655,7 @@ impl<'a, 'b> BundleSpawner<'a, 'b> {
  self.change_tick,
  bundle,
  );
- self.entities.meta[entity.id as usize].location = location;
+ self.entities.meta[entity.index as usize].location = location;
 
  location
  }