Fix block kit link & update docs

[clavin]

Summary

Fixes a single link to Block Kit & updates the docs to include the 2.1.0 changelog.

Future Work

Maybe there should be a section about the message builder in v2.1 😯

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #484 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #484   +/-   ##
=======================================
  Coverage   69.58%   69.58%           
=======================================
  Files          15       15           
  Lines        1654     1654           
  Branches       91       91           
=======================================
  Hits         1151     1151           
  Misses        481      481           
  Partials       22       22

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