ENH improve HTML display of FunctionTransformer

[Charlie-XIAO]

Fixes #29032.

Main changes explained

• Add a caption field to the label which is the smaller, grayed text shown below the name.
• Replace FunctionTransformer in the name field by the function name and display the estimator name in caption; the function name is est.func.__name__, or est.func.func.__name__ (for functools.partial), or {est.func.__class__.__name__}(...).
• Allow Documentation to {xxx} tooltip to show something other than xxx=name; this is needed in this case because name is not the estimator name now.
• Change the label to flex layout which seems to me more suitable and flexible in this case

This is more or less still a draft solution so discussions/suggestions welcome.

Testing this PR

In a notebook:

from functools import partial

import numpy as np
from sklearn.pipeline import make_pipeline
from sklearn.preprocessing import FunctionTransformer

def function_one(x, y, z):
    return x + y + z

pipe = make_pipeline(
    FunctionTransformer(lambda x: x + 1),
    FunctionTransformer(function_one),
    FunctionTransformer(partial(function_one, y=1)),
    FunctionTransformer(partial(partial(function_one, y=1), z=2)),
    FunctionTransformer(np.vectorize(function_one)),
    FunctionTransformer(np.vectorize(partial(function_one, y=1))),
)
pipe

The above would render:

[github-actions]

✔️ Linting Passed

All linting checks passed. Your pull request is in excellent shape! ☀️

_{Generated for commit: 77d2a24. Link to the linter CI: here}

[adrinjalali]

I like the idea!

[Charlie-XIAO]

I'm not sure why codecov is failing for this PR. The only uncovered lines in the report did not seem related to this PR, or should I fix the partially covered lines as well?

[adrinjalali]

This LGTM.

[Charlie-XIAO]

@timvink @glemaitre Would you want to take a look at this one?

[timvink]

Yes this look great !

[adrinjalali]

Merge conflict, also, maybe @lesteve could have a look here?

[Charlie-XIAO]

Changelog conflict, resolved :)

[thomasjpfan]

Small nit, otherwise LGTM

[Charlie-XIAO]

Thanks for the review @thomasjpfan; comment resolved in 77d2a24