docs: Update workflow syntax with priority directive

[bentyeh]

Description

1. Add priority directive to Snakefile syntax specification (see https://snakemake.readthedocs.io/en/stable/snakefiles/writing_snakefiles.html)
2. Add Wikipedia link for extended Backus-Naur form (EBNF).

QC

• The PR contains a test case for the changes or the changes are already covered by an existing test case.
• The documentation (docs/) is updated to reflect the changes or this is not necessary (e.g. if the change does neither modify the language nor the behavior or functionalities of Snakemake).

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[bentyeh]

There are actually many directives that are supported by Snakemake but not currently included in the Snakefile syntax specification documentation: ruleorder, localrules, onsuccess, onerror, onstart, etc.

Unfortunately, I'm not sufficiently familiar with Snakemake to know all the supported directives. Would @johanneskoester or another more senior contributor be able to complete the syntax specification documentation with all supported directives?

Finally, perhaps it would be helpful to new users to define what the word "directive" means. The current documentation seems to use "directive" to refer to any keyword that is followed by a colon in a Snakefile. Is that accurate?

[johanneskoester]

There are actually many directives that are supported by Snakemake but not currently included in the Snakefile syntax specification documentation: ruleorder, localrules, onsuccess, onerror, onstart, etc.

Unfortunately, I'm not sufficiently familiar with Snakemake to know all the supported directives. Would @johanneskoester or another more senior contributor be able to complete the syntax specification documentation with all supported directives?

Finally, perhaps it would be helpful to new users to define what the word "directive" means. The current documentation seems to use "directive" to refer to any keyword that is followed by a colon in a Snakefile. Is that accurate?

Yes, that is correct. And indeed, the EBNF should be updated. I hope to find time for that soon.

[bentyeh]

Referencing related open issues #1502 #1168 #2224