Improve vendoring process and `Makefile-cigraph`

[krlmlr]

• Update all redundant files using just make -f Makefile-cigraph

  Some components (flex, bison) in the Makefile need specific versions of the tools, which is why we advise to run make in Docker. But we actually don't need to run the entire Makefile, running only the flex and bison parts in Docker is sufficient.

• Avoid Docker dependency: chore: Fix drift after code generation #1278 (comment)

  Should be optional, perhaps omitting flex and bison under some conditions?

• More precise rules

  Running make -f Makefile-cigraph should "just work" in more cases, even without the -B switch. These days, often it seems we need -B because changes are not detected

• Include other tooling into the Makefile

  Example: updating the vendored sources

• Fix make docs and other potential broken rules (make docs fails #913)

• Clean up documentation in src/README.md