Move wheel building from Travis to GitHub Actions

[luizirber]

This PR completely removes Travis from our CI setup.

• Use https://cibuildwheel.readthedocs.io/en/latest/faq/#emulation to build aarch64 wheels
• Rename .travis/ to .ci
• Update docs to point to GH Actions instead of Travis
• Split wheel building into many jobs (so it doesn't take forever)

This also sort of setup the way for universal2/arm64 wheels on macOS (once it is more stable).

Checklist

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[codecov]

Codecov Report

Merging #1295 (f6515bc) into latest (fe5d210) will increase coverage by 5.26%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           latest    #1295      +/-   ##
==========================================
+ Coverage   88.69%   93.96%   +5.26%     
==========================================
  Files         125       98      -27     
  Lines       18281    14665    -3616     
  Branches     1440     1440              
==========================================
- Hits        16215    13780    -2435     
+ Misses       1819      638    -1181     
  Partials      247      247              

Flag	Coverage Δ
python	93.96% <ø> (ø)
rust	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/core/src/errors.rs
src/core/src/lib.rs
src/core/src/cmd.rs
src/core/src/signature.rs
src/core/src/ffi/hyperloglog.rs
src/core/src/ffi/signature.rs
src/core/src/index/bigsi.rs
src/core/src/encodings.rs
src/core/src/ffi/nodegraph.rs
src/core/src/sketch/nodegraph.rs
... and 17 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update fe5d210...f6515bc. Read the comment docs.

[luizirber]

Ready for review and merge @ctb