Remove examples of time_imports nesting given it was a bug

[IanButterworth]

As noted in #45861 (comment) the nested loading was a bug that #45861 fixed.

That means that the nesting in @time_imports is no longer seen, so this updates the docs. Also fixes up the changelog, which needs manual backport.

Note that it means that the times shown are purely the import time of each package now (not the sum of all nested times), given that its deps are loaded further up the order.

So, previously, something like

julia> @time_imports using CSV
      0.4 ms    ┌ IteratorInterfaceExtensions
     11.1 ms  ┌ TableTraits 84.88% compilation time
    145.4 ms  ┌ SentinelArrays 66.73% compilation time
     42.3 ms  ┌ Parsers 19.66% compilation time
      4.1 ms  ┌ Compat
      8.2 ms  ┌ OrderedCollections
      1.4 ms    ┌ Zlib_jll
      2.3 ms    ┌ TranscodingStreams
      6.1 ms  ┌ CodecZlib
      0.3 ms  ┌ DataValueInterfaces
     15.2 ms  ┌ FilePathsBase 30.06% compilation time
      9.3 ms    ┌ InlineStrings
      1.5 ms    ┌ DataAPI
     31.4 ms  ┌ WeakRefStrings
     14.8 ms  ┌ Tables
     24.2 ms  ┌ PooledArrays
   2002.4 ms  CSV 83.49% compilation time

Now:

julia> @time_imports using CSV
     50.7 ms  Parsers 17.52% compilation time
      0.2 ms  DataValueInterfaces
      1.6 ms  DataAPI
      0.1 ms  IteratorInterfaceExtensions
      0.1 ms  TableTraits
     17.5 ms  Tables
     26.8 ms  PooledArrays
    193.7 ms  SentinelArrays 75.12% compilation time
      8.6 ms  InlineStrings
     20.3 ms  WeakRefStrings
      2.0 ms  TranscodingStreams
      1.4 ms  Zlib_jll
      1.8 ms  CodecZlib
      0.8 ms  Compat
     13.1 ms  FilePathsBase 28.39% compilation time
   1681.2 ms  CSV 92.40% compilation time

(These timings haven't been tested on the same CSV & dep versions. They shouldn't be taken to mean there's a load speed improvement)