Document other meta-analysis tools outside our ecosystem

[tsalo]

Closes #503.

Changes proposed in this pull request:

• Add a section to the About NiMARE page about tools outside our ecosystem.

[codecov]

Codecov Report

Merging #654 (1465729) into main (0c85370) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #654   +/-   ##
=======================================
  Coverage   85.25%   85.25%           
=======================================
  Files          40       40           
  Lines        4530     4530           
=======================================
  Hits         3862     3862           
  Misses        668      668           

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