Correctly set docs version number for release workflow

[AdrianoKF]

The github.ref_name context property appears to no longer be set when making a release through the UI. This PR applies a workaround, using the github.event.release.tag_name property instead.

See actions/runner#2788 for details.

Issue: #227

[codecov[bot]]

Codecov Report

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

Comparison is base (8ad9706) 90.56% compared to head (e1c25a0) 90.56%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #228 +/- ## ======================================= Coverage 90.56% 90.56% ======================================= Files 7 7 Lines 551 551 Branches 93 93 ======================================= Hits 499 499 Misses 27 27 Partials 25 25 ```

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