docs(project-nvim): add note about built-in functionality

[rocode]

📑 Description

Adds a brief note to the project-nvim README.md file that AstroNvim contains a built-in Project Rooter, and if the community plugin is enabled, the built-in functionality will be disabled.

ℹ Additional Information

This PR originated from a discord conversation in the #astrocommunity channel.

@rocode: Should community plugins have a note on them if their functionality was already upstreamed or incorporated into AstroCore? For example, I just realized I didn't need project-nvim after seeing the Project Rooter recipe in the docs, but trying to migrate over caused confusion because it isn't documented that it turns off the built-in functionality. [...] @mehalter: yeah that would be great! If you open a PR it would be good to note on that plugin (and maybe the other project plugins) that there is a new rooter in AstroNvim that is on demand by default and opt in for automatic root changing

[github-actions[bot]]

Review Checklist

Does this PR follow the [Contribution Guidelines](development guidelines)? Following is a partial checklist:

Proper conventional commit scoping:

• If you are adding a new plugin, the scope would be the name of the category it is being added into. ex. feat(utility): added noice.nvim plugin

• If you are modifying a pre-existing plugin or pack, the scope would be the name of the plugin folder. ex. fix(noice-nvim): fix LSP handler error

• [x] Pull request title has the appropriate conventional commit type and scope where the scope is the name of the pre-existing directory in the project as described above

• [x] README is properly formatted and uses fenced in links with <url> unless they are inside a [title](url)

• [x] Proper usage of opts table rather than setting things up with the config function.