[builder] Add objectness scores

[felixdittrich92]

This PR:

• unify detection predictor output (N, 5) / (N, 5, 2) --> last is objectness_score
• add objectness score to Document output
• update API template
• update docs
• add corresponding tests

Use case: enables the end user to filter out some detected artefacts for example

Any feedback is welcome :hugs:

NOTE: API tests will fail until merged :)

Related to: #1612

[codecov[bot]]

Codecov Report

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

Project coverage is 96.39%. Comparing base (5eda559) to head (52a6850).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1625 +/- ## ======================================= Coverage 96.39% 96.39% ======================================= Files 164 164 Lines 7716 7744 +28 ======================================= + Hits 7438 7465 +27 - Misses 278 279 +1 ``` | [Flag](https://app.codecov.io/gh/mindee/doctr/pull/1625/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mindee) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/mindee/doctr/pull/1625/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mindee) | `96.39% <100.00%> (+<0.01%)` | :arrow_up: | 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=mindee#carryforward-flags-in-the-pull-request-comment) to find out more.

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

[felixdittrich92]

ref.: #1612