Issue #425 - Updating package documentation

[nikosbosse]

Description

This PR closes #425. This PR closes ##702.

This PR

• Starts to reformat overall documentation to comply with the tidyverse style guide. Made function titles sentence cases and added a dot at the end of each sentence. But phew... I might have missed a few instances. Also who knows where the styleguide wants you to make a linebreak and where not? Sometimes it's still a bit confusing (e.g. what should the order of the @ args be? I think it's a step in the right direction, but the path is long and cumbersome
• Updates the documentation for as_forecast() in particular, adding the table from the paper with the different formats. Currently, both as_forecast() and score() have that info. We could delete it from score() or leave it there. Any opinions? (I also added some duplicated info to get_forecast_type() and get_forecast_unit(), but I think that's helpful..
• Makes all kinds of minor updates in various places (adding dots, linebreaks etc.)

I will have to update the inputs to quantile_score() once again, but I'll do that when I get to working on the input checks (#748).

Here are several emotional damage platypus:

And a baby toucan for good measure:

Checklist

• [x] My PR is based on a package issue and I have explicitly linked it.
• [x] I have included the target issue or issues in the PR title as follows: issue-number: PR title
• [x] I have tested my changes locally.
• [x] I have added or updated unit tests where necessary.
• [x] I have updated the documentation if required.
• [x] I have built the package locally and run rebuilt docs using roxygen2.
• [x] My code follows the established coding standards and I have run lintr::lint_package() to check for style issues introduced by my changes.
• [x] I have added a news item linked to this PR.
• [x] I have reviewed CI checks for this PR and addressed them as far as I am able.

[codecov[bot]]

Codecov Report

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

Project coverage is 95.30%. Comparing base (a3c70c9) to head (9521f84).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #747 +/- ## ======================================= Coverage 95.30% 95.30% ======================================= Files 21 21 Lines 1596 1596 ======================================= Hits 1521 1521 Misses 75 75 ```

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[nikosbosse]

This PR turned out to be a bit of a monster, unfortunately. If you like I'm happy to split it up a bit. That wouldn't change the overall quantity, but maybe it's nicer to review.