[Feature]: Source clone and build from the binary

I was fixing up the man docs because pandoc has issues dealing with markdown used in the website and saw that there's a compilation guide in the man documentation.

This doesn't make sense because if someone already has a built version of conky, then they likely don't need this info.

Then I figured "what if it _made sense_" - conky could provide an argument that:
- clones conky from the git repo
- automatically starts `ccmake`, with current build flags enabled
- builds a modified flavor of conky and installs it

This would be a very convenient way of allowing people to modify their existing builds. There are however a few problems with it:
- it would have to be able to deal with system not having prerequisites installed.
- how does it go about updating package manager information?
- current CMake doesn't support [hermetic builds](https://github.com/brndnmtthws/conky/issues/2135), so the process would have to be restarted several times if some (additional) libraries are missing.
- if the package manager installed some dependencies, it wouldn't be able to tell they're no longer needed.

Start with the man documentation's compilation guide and the existing CMake/ccmake workflow mentioned in the issue. Clarify the supported clone, dependency, package-manager, rebuild, and install behavior before locating implementation entry points. Done means the scope and failure handling are agreed and the full workflow is demonstrably reliable.

Written by the indexing model from the issue text.