🖊️ Fix filename and highlight typo in contribute-add-feature.md

[ValMystletainn]

Change

the code block filename and emphasize-lines in docs/contribute-add-feature.md,
turns some packages/myst-transforms/src/index.ts into packages/myst-cli/src/process/mdast.ts to keep the consistency with the context above and practice.

The previous version just mislead me at my first time to go through the guide

[changeset-bot]

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[agoose77]

This is a great catch! Thanks :)