Docs: Add @source alias

[peteruithoven]

Summary
Add @source webpack alias to docs. This is can be useful for people who prefer to refer to files from the source folder down instead of a relative folder.
More background: #2308

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

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Does this PR introduce a breaking change? (check one)

• Yes
• No

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The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

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If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[peteruithoven]

Anything I can improve?
I edited te name of the PR for the Semantic Pull request bot, but I can't trigger a re-check.

[ulivz]

LGTM