docs(core): add description to nx-plugin package

[bcabanes]

Adds description to nx-plugin package.

[nx-cloud]

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✅ Successfully ran 6 targets

• nx affected --target=e2e --base=5d281622693f2107eabd309dcb7dcbf644ca49aa --head=e7fb6f8f91819d48af9aa9b1e67e25dc08b14d27 --exclude=e2e-storybook,e2e-react-native,e2e-detox --parallel=1
• nx run-many --target=build --all --parallel=8 --exclude=docs,nx-dev-data-access-documents,nx-dev-e2e,nx-dev,nx-dev-feature-analytics,nx-dev-feature-conf,nx-dev-feature-doc-viewer,nx-dev-feature-search,nx-dev-feature-storage,nx-dev-feature-versions-and-flavors,nx-dev-ui-commands,nx-dev-ui-common,nx-dev-ui-home,nx-dev-ui-member-card,nx-dev-ui-sponsor...
• nx affected --target=build --base=5d281622693f2107eabd309dcb7dcbf644ca49aa --head=e7fb6f8f91819d48af9aa9b1e67e25dc08b14d27 --parallel=3
• nx affected --target=test --base=5d281622693f2107eabd309dcb7dcbf644ca49aa --head=e7fb6f8f91819d48af9aa9b1e67e25dc08b14d27 --parallel=1
• nx affected --target=lint --base=5d281622693f2107eabd309dcb7dcbf644ca49aa --head=e7fb6f8f91819d48af9aa9b1e67e25dc08b14d27 --parallel=3
• nx build typedoc-theme

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