Recommend `mean((x, y))` rather than `middle((x, y))`

[nalimilan]

It seems more logical and simpler for users to recommend using the same function. This also helps making Statistics a standalone package (JuliaStats/Statistics.jl#128) as mean will be moved to Julia Base, but middle will remain in Statistics.

This blocks https://github.com/JuliaLang/julia/pull/46501.

See https://github.com/JuliaStats/Statistics.jl/pull/131. Cc: @LilithHafner

[nalimilan]

Apparently this won't be required to make Statistics an upgradable stdlib, but it's probably a good idea anyway.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01% :tada:

Comparison is base (14438ab) 96.99% compared to head (ac7349f) 97.01%. Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #147 +/- ## ========================================== + Coverage 96.99% 97.01% +0.01% ========================================== Files 1 1 Lines 433 435 +2 ========================================== + Hits 420 422 +2 Misses 13 13 ``` | [Files Changed](https://app.codecov.io/gh/JuliaStats/Statistics.jl/pull/147?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaStats) | Coverage Δ | | |---|---|---| | [src/Statistics.jl](https://app.codecov.io/gh/JuliaStats/Statistics.jl/pull/147?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaStats#diff-c3JjL1N0YXRpc3RpY3Muamw=) | `97.01% <100.00%> (+0.01%)` | :arrow_up: |

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