Creating table of content with Graphviz tool for IPCC reports navigation

[rkclimate20]

Add small descriptions and links to the code and output from the repository

[sarthh7777]

the description can be viewed in "svg_crosschapters.md", and the code has been uploaded in the file "AR6_crosschapters_code.ipynb"

, the output graph is "AR6_updated.svg"