Add some more detail to the build from source section in the RPC admin guide

[johncanneto]

https://developers.stellar.org/docs/data/rpc/admin-guide#build-from-source

[mollykarcher]

I think we should the "Build from Source" content in Github and just link to it, as it's currently set up. The audience of the Developer Docs are developers trying to use our products/tools to build their own applications. The audience of the Github READMEs/repos are developers that are trying to contribute to our open-source repositories. The latter is the primary audience for building code from source, since you're typically trying to amend/add to it in some way.

I will also add, if you're building code from source, you have a high-level of awareness that you're doing the "hard" thing and fully expect to be interacting with a Github README rather than a docs page. To even be able to build from source, you need to visit the Github where the source code lives, so I think it would feel really clunky to have to be back-and-forthing between the Github and the docs page in this scenario.