docs: introduce an overview page on each section

[profanis]

This PR introduces an 'Overview' page below each section.

In the GitBook CMS, an icon appears near the Overview page. I think that this appears only in the edit mode. If eventually, this appears even when we merge, we could revert the merge commit.

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BundleMon (Integration Projects)

Unchanged files (2)

Status | Path | Size | Limits :------------: | ------------ | :------------: | :------------: :white_check_mark: | Main bundles(Gzip)
_{hello-world-ng17/dist-integration/main.(hash)
.js}
| 68.48KB | +1% :white_check_mark: | Main bundles(Gzip)
_{hello-world-ng16/dist-integration/main.(hash)
.js}
| 67.03KB | +1%

No change in files bundle size

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Current branch size history | Target branch size history

[codeclimate[bot]]

Code Climate has analyzed commit bec599d6 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (50% is the threshold).

This pull request will bring the total coverage in the repository to 95.2% (0.0% change).

View more on Code Climate.