Fix broken links in docs

[KyrietS]

What does this PR do?

Fix broken links in docs (#1634)

How does this PR change Premake's behavior?

no

Anything else we should know?

• I've added some additional information to website/README about referencing other pages in docs.

Adding a reference to another documentation page

Always reference another documentation page like this:

[some text](Case-Sensitive-Filename.md)

and never like this:

[some text](some-markdown-file)
[some text](/docs/some-markdown-file)
[some text](https://premake.github.io/docs/some-markdown-file)

• As always I'm hosting a working demo to prove that this PR fixes the issue:
  https://kyriets.github.io/premake-core/docs/Developing-Modules

closes #1634

Did you check all the boxes?

• Focus on a single fix or feature; remove any unrelated formatting or code changes
• Add unit tests showing fix or feature works; all tests pass
• Mention any related issues (put closes #XXXX in comment to auto-close issue when PR is merged)
• Follow our coding conventions
• Minimize the number of commits
• Align documentation to your changes

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[samsinsane]

Unrelated, but we should update wording like this. It reads more like a community review, than a statement from the development team. @starkos

[starkos]

Thanks for taking care of this!