[REF] Add major classes/functions to parent namespaces

[tsalo]

Closes #388.

Changes proposed in this pull request:

• Add major classes and functions to parent module namespaces.

[codecov]

Codecov Report

Merging #600 (60def09) into main (bdcec80) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #600      +/-   ##
==========================================
+ Coverage   85.73%   85.76%   +0.02%     
==========================================
  Files          40       40              
  Lines        4382     4390       +8     
==========================================
+ Hits         3757     3765       +8     
  Misses        625      625              

Impacted Files	Coverage Δ
nimare/annotate/__init__.py	100.00% <100.00%> (ø)
nimare/decode/__init__.py	100.00% <100.00%> (ø)
nimare/meta/__init__.py	100.00% <100.00%> (ø)

---

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 bdcec80...60def09. Read the comment docs.

[tsalo]

The Sphinx interlinking issue is still there (and apparently has been for a while without me noticing), but the circular import issue seems to be resolved, so I'm going to merge this.