Update the docs

Remove all mentions of pkg-config

Remove installation section, including discussion of packages.

After this, the "intro/install.rst" becomes a poor name.  I'll
rename it in a followup patch.

gcc/jit/ChangeLog.jit:
	* docs/intro/install.rst ("Installation via packages"): Drop
	this section.
	("Installation from source"): Drop this section, moving parts
	of it to https://gcc.gnu.org/wiki/JIT and some others to
	docs/internals/index.rst.
	("Hello world"): This section becomes the only remaining part
	of this file.  Eliminate references to pkg-config.

	* docs/internals/index.rst
	("Using a working copy without installing every time"): Rewrite
	as...
	("Working on the JIT library"): ...new section, aimed at
	contributors (and myself) working from a build directory,
	eliminating references to installation.  Add description
	of pertinent configuration options.
	("Running the test suite"): Add setting of LIBRARY_PATH to
	description of how to run a built binary outside of the test
	suite.
	("Environment variables"): New section, describing pertinent
	environment variables.
Renumber the tutorial

Rename "install.rst" to "tutorial01", renumbering the other tutorial
sections, making analogous renumberings to the examples.

gcc/jit/ChangeLog.jit:
	* docs/intro/index.rst: Drop install.rst.  Add tutorial04.rst.
	* docs/intro/install.rst: Rename to...
	* docs/intro/tutorial01.rst: ...this, renaming old tutorial01.rst to...
	* docs/intro/tutorial02.rst: ...this, renaming old tutorial02.rst to...
	* docs/intro/tutorial03.rst: ...this, renaming old tutorial03.rst to...
	* docs/intro/tutorial04.rst: ...this.
	* docs/examples/install-hello-world.c: Rename to...
	* docs/examples/tut01-hello-world.c: ...this.
	* docs/examples/tut01-square.c: Rename to...
	* docs/examples/tut02-square.c: ...this.
	* docs/examples/tut02-sum-of-squares.c: Rename to...
	* docs/examples/tut03-sum-of-squares.c: ...this.
	* docs/examples/tut03-toyvm: Rename directory to...
	* docs/examples/tut04-toyvm: ...this.
	* docs/examples/tut04-toyvm/toyvm.c (PATH_TO_SCRIPTS): Update
	for directory renaming.