Split out `as_forecast()`

[nikosbosse]

Description

This PR closes #951.

It

• removes the as_forecast man page
• documents the required input formats in the respective as_forecast_<type> man pages and gets rid of a centralised "here is an overview of all input formats" file
• fixes a few out-of-date things
• adds a figure with input formats for nominal forecasts

Checklist

• [ ] My PR is based on a package issue and I have explicitly linked it.
• [ ] I have included the target issue or issues in the PR title as follows: issue-number: PR title
• [ ] I have tested my changes locally.
• [ ] I have added or updated unit tests where necessary.
• [ ] I have updated the documentation if required.
• [ ] I have built the package locally and run rebuilt docs using roxygen2.
• [ ] My code follows the established coding standards and I have run lintr::lint_package() to check for style issues introduced by my changes.
• [ ] I have added a news item linked to this PR.
• [ ] 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 99.02%. Comparing base (732b22e) to head (acafaee). Report is 8 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #955 +/- ## ========================================== - Coverage 99.32% 99.02% -0.30% ========================================== Files 22 31 +9 Lines 1618 1633 +15 ========================================== + Hits 1607 1617 +10 - Misses 11 16 +5 ``` | [Flag](https://app.codecov.io/gh/epiforecasts/scoringutils/pull/955/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=epiforecasts) | Coverage Δ | | |---|---|---| | [](https://app.codecov.io/gh/epiforecasts/scoringutils/pull/955/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=epiforecasts) | `?` | | 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=epiforecasts#carryforward-flags-in-the-pull-request-comment) to find out more.

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