[doc/developing] added `The repository structure` to the index
[doc/developing] added `The repository structure` to the index

Signed-off-by: Akos Kitta <5adc53eb8928d8424ddf49bf102dba40aa33cffe@gmail.com>
hexa00/theia,hexa00/theia,lmcgupe/theia,epatpol/theia-1,epatpol/theia-1,lmcgupe/theia,epatpol/theia-1