Add search scope availability and permissions to docs

What does this MR do and why?

This MR introduces Search to the role and permissions page. The Search feature is a cross-functionality feature that has the same permission/role gating as the features which are searchable. However, it's not documented anywhere - this is a first step towards documenting what roles are allowed to use search for the indexed data.

This MR also introduces a new search page that details when scopes are available for users in basic, advanced, and exact code search.

AI Summary

This update expands the permissions documentation by adding search capabilities across different project features. The changes add new rows to the permissions tables showing which user roles (Guest, Planner, Reporter, Developer, Maintainer, Owner) can search issues, confidential issues, milestones, tasks, OKRs, and wikis. The formatting of the tables has also been improved for better readability, with consistent column widths and alignment. No actual permission changes were made - this is purely a documentation enhancement to clarify which roles have search capabilities for different project elements.

References

• Support Planner role in Advanced search (&17674)
• Add reference topic with search and scope info ... (#526582 - closed)

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How to set up and validate locally

docs only

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