Clarify GitHub Pages base config and withastro/action usage

[casungo]

Description

This PR updates the English documentation for deploying Astro sites to GitHub Pages (guides/deploy/github.mdx) to address several points of confusion and provide clearer, more accurate instructions.

These changes aim to significantly reduce confusion for users deploying to GitHub Pages subpaths and improve the reliability of their setups by promoting best practices for path handling.

Related issues & labels

• Closes (en) github.mdx: The description of the text is discrepancy. #10868
• Suggested label: improve or update documentation

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Warnings reference

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[sarah11918]

Hi @casungo ! Sorry this PR hasn't had much activity on it! I was hoping someone would be available to help go through this!

Firstly, there are conflicts to resolve. Would you mind handling those?

Secondly, the linked issue mentions that this did not fix someone's problem, but linked to a blog post showing adding a nojekyll file did, and I know that has been a solution to people in the past. I don't see this updated for that extra information in the issue, so I think it would be important to add that here.

Since I haven't had a chance to carefully go through this, I do want to first only update what is incorrect so that these instructions lead to success for someone. (Vs changing explanations / descriptions because they might be "better"). It is much easier to review a PR that only includes corrections for things that are wrong than one that also tries to improve it in various ways. And, since we're struggling to keep up with PRs right now, we need all the help we can get! 😅

So, can we make sure this PR is only about correcting wrong or missing info, so that we can this reviewed and merged more quickly? For example, this PR changes statements that are correct like:

You can set up a custom domain by adding the following ./public/CNAME file to your project:"

to maybe "better" instructions like:

You can set up a custom domain by adding a CNAME file to your project's public/ directory:"

but these don't fix a problem with incorrect statements at this time. I don't disagree that your statement is better! But, there are many of these "upgrades" that don't fix a problem, and that makes it harder to spot the things that actually are incorrect, and make this PR take longer to review. 😅 Right now, we really need to optimize for correcting inaccuracies!

Would you be willing to rework this PR in the above ways? (Resolve conflicts, figure out how to helpfully add the nojekyll information, and only focus on correcting things that are wrong/missing statements?) That would be super helpful, and we could get moving on this! (And, a later PR with "improvements" would always be welcome, where we can concentrate only on language, knowing that the basic steps all work!)