docs: Update RULES.md to include NOTICES.md information & delete NOTICES.md

[isabelle-dr]

Closes #1006
Closes #1154

Summary:
Following the discussions in PR #1127 (comment), the objective of this PR is to provide a better user experience when reading the validation report. This is part of the effort of the 3.1.0 release.
The HTML report in #1127 will link to the corresponding piece of documentation using the anchor links, in order to ease the process of (1) understanding what the issue is, and (2) fixing the dataset.

This PR contains the following changes:

• the first part of RULES.md has been reworked to make it more useful for a regular user of the validator. The parts that are useful only when contributing have been moved to NEW_RULES.md, which describes how to add a new rule.
• the information available in NOTICES.md has been added to RULES.md using a collapsable feature to make the file more digestible. The information added is:
  • the notice_codes
  • the notice fields description
  • the affected files
• added the notice code into the first table
• some cleanup & small documentation fixes
• NEW_RULES.md has been updated to reflect the change of documentation

Expected outcomes:

• All the documentation that a user needs to understand the validation results & fix their dataset is available in RULES.md
• The user doesn't need to guess the mapping between the CamelCase NoticeName and the snake_case notice_code

Please make sure these boxes are checked before submitting your pull request - thanks!

• Run the unit tests with gradle test to make sure you didn't break anything
• Format the title like "feat: [new feature short description]". Title must follow the Conventional Commit Specification(https://www.conventionalcommits.org/en/v1.0.0/).
• Linked all relevant issues
• Include screenshot(s) showing how this pull request works and fixes the issue(s)

[isabelle-dr]

@maximearmstrong this is ready for review! :)
Do you know why the tests didn't run on this PR?

[maximearmstrong]

Some suggestions for changes in-line before approval. Thank you for this @isabelle-dr!

[isabelle-dr]

I have applied the changes:

• remove notice class to use only notice code
• put the references to the spec outside of the dropdown & add the reference to the spec for notices that were only referring to the python validator
• other small fixes & typos

@barbeau PTAL, the work is done on my end :)

[barbeau]

Thanks for all this work @isabelle-dr! I think it looks much better than before - much easier to read IMHO.

I found a few small typos in the below comments/suggestions.

[isabelle-dr]

What an eye @barbeau! 🧐 Thank you

@maximearmstrong, could you have a last look & approve this PR if it all looks good?

[barbeau]

LGTM, thanks @isabelle-dr!

[maximearmstrong]

2 small things aside, LGTM! Thank you @isabelle-dr :)

Also, we had to reconfigure the CLA assistant last week and I think we will need your signature in order to merge.

[maximearmstrong]

Thank you @isabelle-dr ! The other checks won't be performed because this PR only changes the .md files, so I'm merging it.