jit: doc: Use shared Indices and tables

Apart from that, do not use leading .rst names in toctree.

ChangeLog:

	* doc/indices-and-tables.rst: Rename Indexes to Indices.

gcc/jit/ChangeLog:

	* doc/cp/index.rst: Remove trailing .rst in toctree.
	* doc/cp/intro/index.rst: Likewise.
	* doc/cp/topics/index.rst: Likewise.
	* doc/index.rst: Likewise.
	* doc/intro/index.rst: Likewise.
	* doc/topics/index.rst: Likewise.
	* doc/indices-and-tables.rst: New file.