docs: Add sub-preset examples to the tables in Presets

[TWiStErRob]

Changes

Added sub-preset examples to the tables in Presets in this page: https://docs.renovatebot.com/config-presets/

Context

The syntax is trivial only in hind-sight, it took me a while to figure them out. I had to read the source code to find this magical line which confirmed that this is supported:

renovate/lib/config/presets/util.ts

Line 26 in bf9c495

const [fileName, presetName, subPresetName] = filePreset.split('/');

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[viceice]

we don't like to document, because those are deprecated

[TWiStErRob]

Oh, ok. 3 questions then:

• Is that fact documented somewhere? If not, what is exactly deprecated: subpresets, or json5, or json5 subpresets? (so that I can change the PR to be a warning instead.)
• What is the recommended replacement for the deprecated feature?
• And based on the above, how am I supposed to refactor the structure and references here:
  • Reference: https://github.com/TWiStErRob/renovate-config/blob/c9ccfc7ae99928a853035ae98735e132fb318fff/default.json5#L7
  • Definitions and self-references: https://github.com/TWiStErRob/renovate-config/blob/c9ccfc7ae99928a853035ae98735e132fb318fff/group.json5
  • (2 constraints: same file for easy editing, but separate objects so it's clear what belongs where)

[viceice]

subpresets are soft deprecated, so intentionally not documented.

use path based presets

[HonkingGoose]

@TWiStErRob You should subscribe to this PR:

• docs: clarify preset file name recommendations #28443

Maybe we're dropping the table you're editing in this PR. The maintainers haven't decided yet, but wanted to give a warning about it anyways. 😉