Improvements to WP Plugins section of docs

[benlk]

Changes

This pull request makes the following changes:

• Links out to Helpscout page with list of currently-maintained plugins
• Separates plugin list between maintained and not
• Keeps old-plugin docs around as a historical reference
• Adds new old-plugin docs for more historical references:
  • Developer-Driven Custom Post Classes
  • News Match Donation Shortcode
  • News Match Popup Basics
  • PMP Distribution

Why

For #234

Testing/Questions

Features that this PR affects:

• /projects/wordpress-plugins/ readme

Questions that need to be answered before merging:

• Is this PR targeting the correct branch in this repository?
• Should we try to make this a more-comprehensive list of past and present plugins, with links to all relevant docs including wp.org, Github, support.inn.org, and the status of the plugin?

Steps to test this PR:

1. View https://github.com/INN/docs/tree/234-sunsetting-plugins/projects/wordpress-plugins

[benlk]

Items requested in reviews:

• add https://github.com/INN/developer-driven-custom-post-classes/ to list of maintained plugins
• add notes about other maintained plugins to that list
  • Rivard Donation Plugin https://github.com/INN/rivard-donation-plugin
  • News Match Popup Plugin https://github.com/INN/news-match-popup-plugin
  • PMP Distribution https://github.com/INN/pmp-distribution
  • Trust Indicators https://github.com/INN/trust-indicators
  • NPR Audio Player https://github.com/INN/npr-audio-player
  • INN Members https://github.com/INN/inn-members
  • RNS transmissions https://github.com/INN/rns-transmissions
  • Largo Clean Contact https://github.com/INN/largo-clean-contact
  • Navis Media Credit https://github.com/INN/navis-media-credit
  • Navis Slideshows https://github.com/INN/navis-slideshows
  • No Nonsense Google Analytics https://github.com/INN/no-nonsense-google-analytics
  • Knight Lab Storytelling Tools https://github.com/INN/storytelling-tools
  • Credentials https://github.com/INN/credentials
  • Republication Tracker Tool https://github.com/INN/republication-tracker-tool
  • Client Hosting Manager https://github.com/INN/client-hosting-manager
  • Largo Related Posts https://github.com/INN/largo-related-posts
  • Term Enhancements https://github.com/INN/term-enhancements

[benlk]

• consider deprecated plugins into a subfolder: would this be a way to reduce the depth of this folder?
• if so, remember to look for and update links that point to the moved files, because GitHub won't.

[benlk]

Skipping the subfolder for abandoned/deprecated plugins because at this point the distinction doesn't make sense.

@kaylima this is ready for review and merge, if you'd like to have a sort of oral history of all these plugins available for potential plugin-adopters to peruse.