Add documentation building instructions

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (555a758) 81.97% compared to head (3aa4b2d) 81.97%. Report is 2 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #730 +/- ## ======================================= Coverage 81.97% 81.97% ======================================= Files 41 41 Lines 4149 4149 ======================================= Hits 3401 3401 Misses 748 748 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[yarikoptic]

built of docs failed but for unrelated and fixed since then issues

/home/docs/checkouts/readthedocs.org/user_builds/heudiconv/checkouts/730/README.rst:57: WARNING: image file not readable: figs/environment.png

[github-actions[bot]]

:rocket: PR was released in v1.0.2 :rocket:

[github-actions[bot]]

:rocket: PR was released in v1.0.2 :rocket: