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• fixes a few out-of-date things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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.02%. Comparing base (732b22e) to head (acafaee).
Report is 8 commits behind head on main.

Additional details and impacted files

@@            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     

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[seabbs]

Nice this is much easier to use imo