Extend documentation for relative links

[Marcono1234]

Why:

Closes [issue link]

Extend the documentation for relative links, mentioning that:

• Links are relative to the file in which they appear
• Links starting with / are relative to the repository root

Both of this is currently not explicitly mentioned, but users already rely on it and other documentation sections, such as "Basic writing and formatting syntax" > "Images" make use of it as well.

The wording of the new sentences might not be ideal; any feedback is appreciated.

What's being changed:

see "Why" above

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[ramyaparimi]

@Marcono1234
Thanks so much for opening a PR! I'll get this triaged for review ⚡

[janiceilene]

Thanks so much for opening a PR with this fix 🎉 It looks great! I'll update the branch and get this merged once tests are passing 🍏

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