Introduction user guide

[victortardieu]

Modify the introduction of the user guide so that it doesn't talk about AB4 but Catalogue

[nus-se-pr-bot]

Hi @victortardieu, your pull request title is invalid.

For phase A, it should be in the format of [Learning Outcome ID][Team ID] Your Name, where [Learning Outcome ID] has no dashes or spaces (e.g. [W3.1a]) and [Team ID] has one dash only and no spaces (e.g. [W14-A2] means Wednesday 2pm (14 hrs), Phase A, Team 2).

For phase B, it should be in the format of [v1.x][Team ID] Product Name, replacing v1.x with current milestone progress.

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