DOCS-#7217: Update docs as to when Modin operators work best

[YarShev]

What do these changes do?

• first commit message and PR title follow format outlined here

  NOTE: If you edit the PR title to match this format, you need to add another commit (even if it's empty) or amend your last commit for the CI job that checks the PR title to pick up the new PR title.

• passes flake8 modin/ asv_bench/benchmarks scripts/doc_checker.py
• passes black --check modin/ asv_bench/benchmarks scripts/doc_checker.py
• signed commit with git commit -s
• Resolves Update docs as to when Modin operators work best #7217
• tests passing
• module layout described at docs/development/architecture.rst is up-to-date

[dchigarev]

Suggestion for groupby section:

This operator performs best when the cardinality of 'by' columns is low (small number of output groups). At the Map stage, the operator computes the aggregation for each row partition individually, meaning, that the Reduce stage takes a dataframe with the following number of rows: num_groups * n_row_parts. If the number of groups is too high, there's a risk of getting a dataframe with even bigger than the initial shape at the Reduce stage.

Example:

initial_nrows = 1_000
num_groups = 800
num_row_parts = 16

Reduce stage input shape: num_groups * num_row_parts = 800 * 16 = 12_800 rows (12 times bigger than the initial shape,
                                                                               even full-column implementation would have done better)