Link top level names in class hierarchy with intersphinx when possible.

[tristanlatr]

Tackle part of issue #310. But other cases still needs to be taken care of.

[codecov[bot]]

Codecov Report

Patch coverage: 83.33% and project coverage change: +0.08 :tada:

Comparison is base (1403989) 92.51% compared to head (f9ef4e3) 92.60%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #709 +/- ## ========================================== + Coverage 92.51% 92.60% +0.08% ========================================== Files 47 47 Lines 8126 8129 +3 Branches 1942 1942 ========================================== + Hits 7518 7528 +10 + Misses 354 345 -9 - Partials 254 256 +2 ``` | [Impacted Files](https://app.codecov.io/gh/twisted/pydoctor/pull/709?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=twisted) | Coverage Δ | | |---|---|---| | [pydoctor/templatewriter/summary.py](https://app.codecov.io/gh/twisted/pydoctor/pull/709?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=twisted#diff-cHlkb2N0b3IvdGVtcGxhdGV3cml0ZXIvc3VtbWFyeS5weQ==) | `91.91% <75.00%> (+3.17%)` | :arrow_up: | | [pydoctor/linker.py](https://app.codecov.io/gh/twisted/pydoctor/pull/709?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=twisted#diff-cHlkb2N0b3IvbGlua2VyLnB5) | `90.07% <100.00%> (-0.07%)` | :arrow_down: |

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[tristanlatr]

I’ll merge this PR next week if there is still no review