Rename bracket-push exercise to matching-brackets

[thomchop]

Resolves #379

This rename was proposed in: exercism/problem-specifications#1501

The rationale for the name is:

• to name the exercise by its story, not by what it potentially teaches
• to avoid unnecessarily biasing the solution space

[iHiD]

Thanks a lot for this!

Once this is approved, can we leave @ErikSchierboom to merge it please, so we can check it all goes through properly on the website end. Renaming exercises can be slightly problematic, and I believe I've fixed everything, but it would be good to hand-hold it over the line :)

[angelikatyborska]

There was a change in requirements for practice exercises, and this file now needs to be in .docs/instructions.md instead of README.md (docs: https://github.com/exercism/docs/blob/main/building/tracks/practice-exercises.md#file-docsinstructionsmd)

[iHiD]

@ErikSchierboom Could you double check this one for me pls.

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@thomchop Thanks for doing this. Sorry it took so long to get sorted.
@angelikatyborska Thanks for the nudge to me about what needed sorting :)