change(docs): Update Zebra's build instructions

[upbqdn]

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Motivation

Building Zebra requires the pkg-config tool, which wasn't mentioned in the build instructions. The tool is required by the pkg-config crate here. As part of the solution, I tried to refine the build instructions.

Solution

This PR:

• adds pkg-config to the build dependencies;
• adds a specific example of installing dependencies for Arch;
• renames the Build Instructions section to Building Zebra for consistency with other sections
• moves the Optional Features subsection into the Building Zebra section, and consolidates all Zebra's build features;
• explicitly states that users can combine the features;
• contains a bunch of additional small edits.

Reviewer Checklist

• Will the PR name make sense to users?
  • Does it need extra CHANGELOG info? (new features, breaking changes, large changes)
• Are the PR labels correct?
• Does the code do what the ticket and PR says?
  • Does it change concurrent code, unsafe code, or consensus rules?
• How do you know it works? Does it have tests?

[oxarbitrage]

Changes looks good to me, thanks!