Add NodeHelpDocs in Dynamo Repo

[QilongTang]

Please Note:

1. Before submitting the PR, please review How to Contribute to Dynamo
2. Dynamo Team will meet 1x a month to review PRs found on Github (Issues will be handled separately)
3. PRs will be reviewed from oldest to newest
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7. PRs made to the DynamoRevit repo will need to be cherry-picked into all the DynamoRevit Release branches that Dynamo supports. Contributors will be responsible for cherry-picking their reviewed commits to the other branches after a LGTM label is added to the PR.

Purpose

This is the initial PR

• add Node Help Docs into Dynamo Repo. These are generated locally following https://github.com/DynamoDS/Dynamo/blob/master/src/Tools/NodeDocumentationMarkdownGenerator/readme.md
  an example command:

 .\NodeDocumentationMarkdownGenerator.exe fromdirectory -i "C:\Users\xxx\Documents\GitHub\Dynamo\bin\AnyCPU\Debug" -o "./docs" -w -s -d "C:\Users\xxx\Documents\GitHub\DynamoDictionary\public\data\Dynamo_Nodes_Documentation.json" -x "C:\Users\xxx\Documents\GitHub\Dynamo\src\LibraryViewExtension\web\library\layoutSpecs.json" -f UnitsUI.dll

Binaries covered can be found at Jira task https://jira.autodesk.com/browse/DYN-4001

• a copy step added for DynamoCore.csproj to copy all of the md files and images to fallback_docs folder under DynamoCore folder so that PackageDocumentationManager can search for at https://github.com/DynamoDS/Dynamo/blob/master/src/DocumentationBrowserViewExtension/PackageDocumentationManager.cs#L62. As a result, DynamoCore size after this PR will increase about 85mb.
  

We can create further CI job to update these documents when needed at release stage or by dev request.

Declarations

Check these if you believe they are true

• The codebase is in a better state after this PR
• Is documented according to the standards
• The level of testing this PR includes is appropriate
• User facing strings, if any, are extracted into *.resx files
• All tests pass using the self-service CI.
• Snapshot of UI changes, if any.
• Changes to the API follow Semantic Versioning and are documented in the API Changes document.
• This PR modifies some build requirements and the readme is updated

Release Notes

Add NodeHelpDocs in Dynamo Repo

Reviewers

@DynamoDS/dynamo

FYIs

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[dynamo-ci-user]

Did you use the image compression flag to compress the dictionary images and gifs? This was me (mjk) - I have no idea why my email is apparently registered to the dynamo-ci-bot....

[QilongTang]

@reddyashish Yes I did, let me update the description and give an example command

[mjkkirschner]

@QilongTang reviewing this literally freezes my browser 🙃 - can you add to this PR description which binaries this PR covers?