…s + sample

- update PluginLoader to use load-plugin's native Node module resolution. this library already
  handles loading of files vs modules in node_modules/ so let's use that directly
- add configuration-based plugin loading and deprecate environment variables
- new documentation page for plugin development. loading, writing, etc.
- add PullActionPlugin + unify config on just "plugins"
- add explicit exports to make importing @finos/git-proxy more straight forward for plugins

In addition, this commit adds both CommonJS and ES module examples of plugins and refactors them
into a more reflective package. It also includes updates to the site documentation, specifically
in the Development section, which now includes details about plugins and contribution guidelines.

- fix issue with json-schema-for-humans producing bad output due to post-processing (script step
  no longer required)
- docs: add section on docs on how to update the config schema + re-generate reference doc
- fix: don't package website with git-proxy, update .npmignore on npm pack/publish

- remove actions which are used in internal fork from chain
- add default config value for plugins (an empty array)

- add test cases for getChain + executeChain to validate the processor execution
- remove extraneous function to create a loader and just use the constructor
- address review comments

feat: expose plugin extension points into main package, developer docs + sample

Can't move concurrently because it's used by git-proxy-all at runtime

chore: bump minor for release v1.4.0