docs: replace remaining purgecss by purgeCSSPlugin

[julien-deramond]

Proposed changes

This PR proposes two minor documentation updates to reflect recent changes regarding the use of purgeCSSPlugin. Additionally, it addresses any potential oversights where purgecss was still referenced instead of purgeCSSPlugin.

The modification to docs/getting-started.md appeared straightforward, especially when compared to similar changes we've made in the past. Specifically, we're not utilizing the imported item in this case.

For the update to packages/postcss-purgecss/README.md, I tested the approach locally. However, since I'm not an advanced user of PurgeCSS, there may be nuances I've missed, so I'm open to feedback on this part.

Types of changes

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• Documentation fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

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• Lint and unit tests pass locally with my changes

  • Note: there was a linting issue but apparently not linked to my modifications since I replicate in the mainbranch

  /Users/ju/purgecss/docs/.vuepress/.cache/deps/@vue_devtools-api.js
  3901:5  error  Definition for rule 'es5/no-es6-methods' was not found  es5/no-es6-methods

• (N/A) I have added tests that prove my fix is effective or that my feature works

• I have added necessary documentation (if appropriate)

Further comments

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