Jenkins-YML-WorkflowLibs changes by release

v0.6.0

Moved documentation to being generated from Mkdocs instead of being converted from Markdown to HTML via GitHub pages Jekyll.

Slack: New class com.concur.Slack for sending directly to the Slack API, initially is a drop in replacement for the Jenkins plugin but does not make guarantees about future compatibility.

plNotify: Switch to the new Slack class for sending. Richer notification on end of build.

Git.getVersion: Set increment minor to true by default. This restores the old functionality.

Add SHORT_VERSION to default replacement option, this is the version provided by Git().getVersion() without the build number.

Git.getVersion: Update to be more flexible with tag retrieval, will now get latest from the current branch using git describe --tags
Git.timeSinceLatestTag: Signature changed to accept a tag string that is used in the underlying git command.

Added script to automatically generate documentation on commit.
Update com.concur.* groovy files with documentation.
Updated documentation in vars/*.txt, these are viewable in Pipeline Syntax -> Global Variable Reference from a pipeline job.
Add to setup.groovy script to set Markup formatter to Safe HTML so the vars documentation shows appropriately.

scripts/setup.groovy: Will configure a Jenkins instance to use these workflows/workflow libraries. Including adding as a global library, adding environment variables and adding a SSH credential.
Util.parseChangelog: Reads a changelog file to determine what releases are in it.
GitHubApi.getReleases: Use GitHub's GraphQL API to get information about a repositories releases.
GitHubApi.createRelease: Create a release in GitHub with release notes.

Working towards a more strongly typed system, updated vars from def to a static type in many areas.
Commands.getStageName: Added a default stage name if the method is not available or doesn't return anything.
All: Adding more descriptive assertion/errors so it is easier to determine where an error is happening.
Commands.getCredentialsWithCriteria: Will search global credentials first and return the credential if found before looking through the folder structure.
Commands.getJavaStackTrace: If provided a throwable will return the stacktrace for it as a \n joined string. throwable.getStackTrace() is blocked by Jenkins by default.
Git.runGitShellCommand: Uses powershell or sh step with returnStdout turned on instead of writing a file to the workspace.
Git.getVersion: Updated to remove println or move them to debugPrint.
GitHubApi.githubRequestWrapper: Add missing withCredentials block as well as set the Authorization header based on credentials.
GitHubApi.getPullRequests: Moved to use GraphQL instead of the v3 Rest API.
plNotify: Moved to a closure so instead of calling like this: plNotify(...) it must now be called like this plNotify { ... }.

Ability to load local and remote workflows.
debugPrint method with the ability to control whether to print or not by level.
Error handling when executing at various levels.
Text manipulation including replacements using mustache templating style.
Version generation by looking at existing tags on the repo and generating a semantic compatible version number.
Various helper for setting environment variables for Git information.
Simplified execution using plWorkflow closure.