* docs: add plugin loading spec documentation
The logic for discovering and loading plugins is not well documented on
the current documentation.
This causes issues for users that have to troubleshoot why a particular
plugin cannot be found or installed, so this commit adds a specification
document, detailing what are Packer's expectations when it comes to
discovering plugins.
* Apply suggestions from code review
* Update plugin loading specification navbar
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Co-authored-by: Wilken Rivera <dev@wilkenrivera.com>
