Fix typos and grammar in documentation files

[donatik27]

This PR fixes several typos and improves consistency in documentation files:

1. .circleci/README.md:

• Changed "Dockerhub" to "Docker Hub"
  Reason: Using the official product name for better clarity and consistency.

2. scripts/docker/buildpack-deps/README.md:

• Changed "are" to "is" in "The creation of the images is triggered..."
  Reason: Proper subject-verb agreement for better grammar.

3. test/externalTests/README.md:

• Changed "main" to "the main" in "remove all the branches except for the main one"
  Reason: Added article for proper English grammar and readability.

These changes improve documentation readability and maintain consistent terminology throughout the codebase. The modifications are minor but important for maintaining professional quality in our documentation.

Technical Notes:

• Only documentation files were modified
• No code changes were made
• All changes are grammatical improvements

[github-actions]

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[donatik27]

@cameel
Hi! This PR only contains documentation changes, but the test t_native_compile_ext_pool_together is failing. Could you please take a look if this might be an infrastructure issue? Thank you!