Migrate and improve documentation from old Jenkins Wiki

[vgaidarji]

What has been done

1. Migrated plugin documentation from old Wiki to GitHub. Read more at Plugin Documentation-as-Code: Moving docs to GitHub.
2. Added plugin badges to README using shields.io

• Predefined Jenkins Plugins badge from "Version" section
  
• Predefined Jenkins Plugin Installs badge from "Downloads" category
  
• Dynamic badge for release notes using GitHub JSON API for fetching latest release version https://api.github.com/repos/jenkinsci/plot-plugin/releases/latest and JSON Path for extracting version number $.tag_name
  

3. Mentioned importance of having proper labels in PRs in PR template

How to test

1. Make sure content from old Wiki page is fully migrated to README
2. Perform proof-reading of the README to spot any mistakes
3. Make sure shields.io badges are showing correct data and lead to correct pages when clicked

Checklist

• Git commits follow best practices
• Build passes in Jenkins
• Appropriate tests or explanation to why this change has no tests
• Pull Request is marked with appropriate label (see .github/release-drafter.yml)
• JIRA issue is well described (problem explanation, steps to reproduce, screenshots)
• For dependency updates: links to external changelogs and, if possible, full diffs

[codecov]

Codecov Report

Merging #61 into master will not change coverage by %.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #61   +/-   ##
=========================================
  Coverage     43.38%   43.38%           
  Complexity      190      190           
=========================================
  Files            18       18           
  Lines          1263     1263           
  Branches        192      192           
=========================================
  Hits            548      548           
  Misses          651      651           
  Partials         64       64           

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