Clarify how to run a locally built mirai.

[stonebuddha]

Description

The "Running" section of the developer guide has mixed instructions for using both a locally built binary and a globally installed binary, on both a single file (as rustc) and a crate (via cargo).
It also seems unnecessary to have instructions for globally installed binary, because they are covered somewhere else, and a developer just needs the locally built one during development.

This commit separates the instructions for single files from those for crates, updates all the commands to use a locally built binary, and tweaks the indentation in debug configurations.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• API change with a documentation update
• Additional test coverage
• Code cleanup or just keeping up with the latest Rustc nightly

None of the above: only documentation change.

How Has This Been Tested?

Followed the instructions to build and run locally built mirai on both single files and crates.

[hermanventer]

I presume that you have actually carried out the process documented by this. If so, that is your test plan. If not, please do so.

[stonebuddha]

I presume that you have actually carried out the process documented by this. If so, that is your test plan. If not, please do so.

Yeah, I've followed the process on my machine. I'll update the test plan.