Reformatted manual based on Rust book (2ed.)

manual/src/SUMMARY.md

manual/src/ch01-00-introduction.md

manual/src/ch02-00-features.md

manual/src/ch03-00-miscellaneous.md

manual/src/ch03-01-implicitcd.md

manual/src/ch03-02-xdg.md

manual/src/ch03-03-quotation.md

manual/src/ch03-04-multiargs.md

manual/src/ch03-05-multicomments.md

manual/src/ch03-06-general.md

manual/src/ch04-00-variables.md

manual/src/ch04-01-strings.md

manual/src/ch04-02-tuples.md

manual/src/ch04-03-arrays.md

manual/src/ch04-04-arithmetic.md

manual/src/ch04-05-exporting.md

manual/src/ch05-00-expansions.md

manual/src/ch05-01-variable.md

manual/src/ch05-02-process.md

manual/src/ch05-03-brace.md