[REF, DOC] Make non-user-facing utility functions semi-private and improve docs

[tsalo]

Closes None.

Changes proposed in this pull request:

• In order to simplify the API, I've made a number of internal functions private(ish).
• Remove the roadmap file in the documentation and move the relevant contents to the about file.
• Remove most of the information about the NeuroStore ecosystem from the about file.
• Reorganize examples to be a bit clearer.

[codecov]

Codecov Report

Merging #604 (681f49e) into main (803b5c8) will not change coverage.
The diff coverage is 91.13%.

@@           Coverage Diff           @@
##             main     #604   +/-   ##
=======================================
  Coverage   85.76%   85.76%           
=======================================
  Files          40       40           
  Lines        4390     4390           
=======================================
  Hits         3765     3765           
  Misses        625      625           

Impacted Files	Coverage Δ
nimare/annotate/lda.py	23.75% <33.33%> (ø)
nimare/extract/utils.py	38.58% <50.00%> (ø)
nimare/meta/kernel.py	96.49% <50.00%> (ø)
nimare/decode/continuous.py	95.00% <66.66%> (ø)
nimare/utils.py	94.29% <92.30%> (ø)
nimare/annotate/cogat.py	91.35% <100.00%> (ø)
nimare/dataset.py	89.12% <100.00%> (ø)
nimare/decode/discrete.py	96.72% <100.00%> (ø)
nimare/meta/cbma/base.py	93.67% <100.00%> (ø)
nimare/meta/ibma.py	100.00% <100.00%> (ø)
... and 3 more

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