Extend documentation for the viewers module.

[codecov[bot]]

Codecov Report

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

Comparison is base (91468ba) 82.79% compared to head (3147c23) 82.79%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #515 +/- ## ======================================= Coverage 82.79% 82.79% ======================================= Files 27 27 Lines 4313 4313 ======================================= Hits 3571 3571 Misses 742 742 ``` | [Flag](https://app.codecov.io/gh/aiidalab/aiidalab-widgets-base/pull/515/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidalab) | Coverage Δ | | |---|---|---| | [python-3.10](https://app.codecov.io/gh/aiidalab/aiidalab-widgets-base/pull/515/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidalab) | `82.79% <ø> (ø)` | | | [python-3.8](https://app.codecov.io/gh/aiidalab/aiidalab-widgets-base/pull/515/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidalab) | `82.83% <ø> (ø)` | | | [python-3.9](https://app.codecov.io/gh/aiidalab/aiidalab-widgets-base/pull/515/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidalab) | `82.83% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidalab#carryforward-flags-in-the-pull-request-comment) to find out more.

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