docs(contrib): fix various grammar errors and `markdownlint` issues

[agilgur5]

Summary

There were many grammatical issues in the CONTRIBUTING.md that I noticed and have fixed in this PR. There are also many markdownlint issues, many of which I fixed in this PR

Details

grammar:

• "lot of people" -> "many people"
• incorrect "the" before "backward compatibility", which is not a specific noun
• "shall" -> "should"
• missing comma "," after various prepositions
• fix run-on sentence in volatile style
• etc

markdownlint issues:

• add syntax highlighting to Java codeblock example
• consistently use - instead of * or +
  • all three were used before, only one should be used consistently
• consistently use two space indentation for sub-bullets
  • sometimes it was two space, other times it was four space
• etc

Misc Notes

I was reading through the Contributing guidelines before submitting https://github.com/jenkinsci/github-plugin/pull/404 and noticed some grammatical and other issues in this docs page, so thought I'd fix and improve them while I'm here

Testing done

• markdownlint gives significantly less issues now -- all the lines covered in this PR no longer have errors
• See the rendered markdown rich diff for this PR

Submitter checklist

• [x] Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• [x] Ensure that the pull request title represents the desired changelog entry
• [x] Please describe what you did
• [n/a] Link to relevant issues in GitHub or Jira
• [n/a] Link to relevant pull requests, esp. upstream and downstream changes
• [n/a] Ensure you have provided tests - that demonstrates feature works or fixes the issue