[docs] add Vale as a documentation linter

[odow]

Experimenting again with https://vale.sh.

It found a few typos.

[codecov[bot]]

Codecov Report

Base: 97.63% // Head: 97.62% // Decreases project coverage by -0.00% :warning:

Coverage data is based on head (754f399) compared to base (f3bfb74). Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3080      +/-   ##
==========================================
- Coverage   97.63%   97.62%   -0.01%     
==========================================
  Files          32       32              
  Lines        4305     4303       -2     
==========================================
- Hits         4203     4201       -2     
  Misses        102      102              

Impacted Files	Coverage Δ
src/variables.jl	100.00% <0.00%> (ø)

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[mlubin]

That's a fair number of typos.

[odow]

Maintaining the accept list seems like it could be a pain. But the real issue is that GitHub doesn't have any linting in the web editor, so it's hard to catch these in review.

Anyway. More tooling seems like positive progress if it leads to a higher quality product.

[odow]

This needs a new tag of MOI so we don't fail on the errors in MOI's docs.