Issue #851 - Create S3 methods for converting to point and quantile forecasts

[nikosbosse]

Description

This PR closes #851.

The PR

• creates S3 methods for as_forecast_point() and as_forecast_quantile to convert quantile-based and sample-based forecasts
• splits up the doc files for as_forecast(). ?as_forecast() now has the overview of all the types and there are separate entries for all the specific as_forecast_<type>() functions. In the future we could flesh them out more, but at the moment it's not top on my priority list
• updates tests and the news file.

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

Attention: Patch coverage is 77.77778% with 6 lines in your changes missing coverage. Please review.

Project coverage is 95.20%. Comparing base (4fee240) to head (1904f52). Report is 43 commits behind head on main.

Files	Patch %	Lines
R/forecast.R	76.00%	6 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #876 +/- ## ========================================== - Coverage 95.47% 95.20% -0.27% ========================================== Files 21 21 Lines 1568 1606 +38 ========================================== + Hits 1497 1529 +32 - Misses 71 77 +6 ```

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

[nikosbosse]

I think I successfully addressed all points by deferring them to new issues :D Good to merge?