Add intro to Searching in GitLab

What does this MR do and why?

Adds an intro to Searching in GitLab based on an edited version of this comment

Evaluation

• https://gitlab.com/gitlab-org/technical-writing/-/issues/994#note_1874275993

Related issues

• https://gitlab.com/gitlab-org/technical-writing/-/issues/994
• https://gitlab.com/gitlab-org/technical-writing/-/issues/982

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.