docs(Execution workflow): update output file specifications for reporting region-level 'polyA site usage scores'

Related to #372 & it's PR #373

We have tools like LABRAT, APAlyzer that report their own metrics to represent relative PAS usage within a gene/terminal exon, rather than the relative usage of each individual polyA site within the region. We want to be able consider tools like this for relative quantification benchmarking event.

## Tasks:

- [ ] Update `execution_workflows/execution_output_specification.md` - specify an execution workflow output file format to capture the region-level 'score' for relative PAS usage.
- [ ] Add example output files corresponding to the new specification in `execution_workflows/example_output_files`.

We are currently prioritising the implementation of fractional PAS usage quantification benchmark. For now this issue serves as a reminder to come back to this if we can with references to previous work/discussions!

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### Previous suggestion in PR #373

This generated some discussion and was probably not the consensus view. See [comments](https://github.com/iRNA-COSI/APAeval/pull/373#issuecomment-1179025590) from @dominikburri and [my response](https://github.com/iRNA-COSI/APAeval/pull/373#issuecomment-1183283193).

#### Format 05

This BED file contains positions of regions (e.g. terminal exons of genes, whole genes) and the relative usage values for each identified region in the **score** column.

Fields:

- **chrom** - the name of the chromosome
- **chromStart** - the starting position of the feature in the chromosome; this corresponds to the first nucleotide of the region (e.g. terminal exon, gene); the starting position is 0-based, i.e. the first base on the chromosome is numbered 0
- **chromEnd** - the ending position of the feature in the chromosome; this corresponds to the last nucleotide of the region.
- **name** - defines the name of the identified region. It's recommended to use a conventional identifier (e.g. Ensembl transcript ID, gene ID)
- **score** - relative usage value for the identified region
- **strand** - defines the strand; either "." (=no strand) or "+" or "-".