Enhancement: implemented relevant CI workflows from admiralci

[m-kolomanski]

Issue

Closes #64

Description

Adds .github/workflows/main.yaml file, which will run relevant checks from admiralci repository:

• spellcheck
• lintr
• man pages, checking if documentation is up to date with docstrings
• R CMD check, with note: since we have some warnings already, I have made it so the check will pass even with warnings. In the future, once we clean those up, it would be beneficial to enable the check to fail on warnings too

In addition, the workflow will also run devtools::test().

[Gero1999]

Hello! I saw you need 2 reviewers, not sure if I need to check anything else, but what I can confirm:

✅Re-run of the workflow produce same annotations and checks
✅ Functionalities of the App are kept after file changes (nca.R, general_lineplot.R ...)

[Gotfrid]

Looks good. I've added non-blocking comments/questions.