GH-46475: [Documentation][C++][Compute] Consolidate Acero developer docs

[zanmato1984]

What changes are included in this PR?

1. Move current Acero Developer's Guild under user doc to where under developer docs;
2. Make it the main doc enclosing elaborating docs like Swiss table.

Are these changes tested?

No need.

Are there any user-facing changes?

None.

• GitHub Issue: [Documentation][C++][Acero] Consolidate Acero developer doc #46475

[github-actions]

⚠️ GitHub issue #46475 has been automatically assigned in GitHub to PR creator.

[zanmato1984]

Note this is renamed from "streaming_execution.rst" to "acero.rst" which is more explicit (and shorter). Because "Acero" has become a much more well-known name I assume.

[zanmato1984]

This file is moved from "docs/source/cpp/acero/developer_guide.rst" with two small tweaks:

1. Changing the title from "Developer’s Guide" to "Developing Acero" (which is more aligned with the naming of other docs at the same level).
2. Adding the "Others" section at the bottom as a TOC to enclose sub-docs which are too elaborate to be inlined (currently only swiss_table.rst).

Please note the original "acero.rst" was moved to "acero/swiss_table.rst" (I was thinking about inlining it however found it too long), but git doesn't recognize this pattern and shows many diffs. The actual changes are just the two places mentioned above.

[zanmato1984]

As mentioned in other comment, this is renamed from "developers/cpp/acero.rst".

[zanmato1984]

@github-actions crossbow submit preview-docs

[github-actions]

Revision: 3685f4d

Submitted crossbow builds: ursacomputing/crossbow @ actions-9fea3f3368

Task	Status
preview-docs

[zanmato1984]

There we go.

User doc: before, after (preview).

Dev doc: before (no Acero yet), after (preview).

@raulcd @amoeba @pitrou PTAL. Thanks.