[doc] Fixed typos and broken links in `A quick tour`
[doc] Fixed broken links in `Tutorials`
[doc] Flagged 2 missing sections in "Using TRIQS in your own C++ project"
[doc] Numerous small fixes in 'Green’s functions' (Python part)
[gf] Added 'GfLegendre' and 'MeshLegendre' to __all__ list.
[doc] 2 small fixes in block GF examples.