Merge #10951

10951: feat: assist to generate documentation templates r=Veykril a=numero-744

Closes #10878, #1915 and #4767

Full description is in #10878, better read [the tests](https://github.com/rust-analyzer/rust-analyzer/pull/10951/files#diff-7a64e2efb66b2625443340fcbc96d531baff12c17cc0aaf51885ea94f67de254R424) to understand what this feature does.

- [x] There is one remaining thing about non-`pub` functions, what do you think about it?
- [x] In this PR [empty examples are generated](https://github.com/rust-analyzer/rust-analyzer/pull/10951/files#diff-7a64e2efb66b2625443340fcbc96d531baff12c17cc0aaf51885ea94f67de254R99) for `trait` functions, but maybe no examples should be provided at all.
- [x] If there is already a documentation, add another one with a separator ([currently done](https://github.com/rust-analyzer/rust-analyzer/pull/10951/files#diff-7a64e2efb66b2625443340fcbc96d531baff12c17cc0aaf51885ea94f67de254R74)) or simply disable this assist?
- [x] I will check once more that the generated examples are correct (ie. they are easy to fill before that they are built and tested)

Comments appreciated 😄 

Co-authored-by: Côme ALLART <[email protected]>

Author: bors[bot]