Update plugin loading documentation (hashicorp#12485)

Packer checks a number of directories for plugins upon initialization,
with the introduction of multi-component plugins and underlying changes
to the Packer SDK the ordering changed slightly. These changes update
the related documentation to reflect the new ordering, and adds a plugin
loading ordering section to the docs to help users discover how plugin
loading works.

Include in this change are updates to the PACKER_CONFIG_DIR environment
variables to reflect the XDG base directory specification used as the
default Packer configuration directory layout.

* Update website/content/docs/configure.mdx

version: cut 1.9.1 on VERSION file

cut release 1.9.0

Update CHANGELOG

Cut release 1.8.6

Bump for 1.9.0-alpha release

Cut release 1.8.5

cut release 1.8.4

Cut release 1.8.3

* Fix spacing in CHANGELOG
* Add release date to CHANGELOG

Cut release v1.8.2