[DOCS] Update documentation

[elephaint]

• Updates deprecated frequencies in model examples
• Updates documentation, using utilsforecast.losses instead of nf
• Re-run examples using latest NF version to make sure examples still run and conclusions hold. If not, example adjusted.
• Fixes a number of dead links
• Rewrite of the 'adding model' tutorial to reflect removal of Base* classes
• Small fix in use of MQLoss / HuberMQLoss
• Removed a warning message that's not necessary.

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[marcopeix]

Amazing work! Thanks a lot!