Move Handling Secrets to a separate page, add docs for CredentialsProvider and encrypted strings

[oleg-nenashev]

This is an attempt to improve documentation of the current state before submitting a pull request for #1141 . It should help guide users towards using Credential Providers when feasible, and it also documents current limitations of credentials.

I also used Asciidoc for the new page to have Table of Contents.

Preview: https://github.com/oleg-nenashev/configuration-as-code-plugin/blob/creds-documentation/docs/features/secrets.adoc

Your checklist for this pull request

🚨 Please review the guidelines for contributing to this repository.

• Make sure you are requesting to pull a topic/feature/bugfix branch (right side) and not your master branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or in Jenkins JIRA
• Link to relevant pull requests, esp. upstream and downstream changes
• Did you provide a test-case? That demonstrates feature works or fixes the issue.

[jetersen]

Perhaps a good time to move run locally sometime noise in the main readme

[oleg-nenashev]

@Casz Yes, I was about creating newbiew-friendly tickets for the rest of the documentation. Our front page is too loaded with details

[jetersen]

Sounds reasonable for a good README that isn't too loaded I suggest looking at https://github.com/GitTools/GitVersion

[oleg-nenashev]

Created few issues. If you have other ideas, please create tickets. One issue got implemented in less than 15 minutes. Power of Hacktoberfest 🎉

[oleg-nenashev]

I am going to merge it without waiting for ci.jenkins.io.

[timja]

@gavinfish it would be good if there was an azure plugin for this as well or the azure-credentials plugin supported it

[gavinfish]

@timja Thanks for your information. I have little bandwidth recently and I will put this in backlog temporarily.