docs: change docs to use "apps" instead of "plugins"

[jeronimoalbi]

Resolves #3647

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Visit the preview URL for this PR (updated for commit 1302318):

https://igntservices-docs--pr3651-docs-ignite-apps-tf68c8g6.web.app

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[jeronimoalbi]

If this PR is approved it would require a small refactor to the "CLI plugins API" to change the user facing API from ignite plugin add ... to ignite app install ....

The docs use the concept of Ignite Apps except in some parts that describe the plugin system, but it is explained that under the hood Ignite Apps are implemented using a plugin system based on Hashicorp's plugins. I think is the fastest approach, but we could also decide for a bigger refactor to change the code to also use the concept of app instead of plugin.

[julienrbrt]

lgtm!