docs: Improve documentation for deploying custom plugins

[saurabhmahawar]

Description

Improve and expand the documentation for deploying custom plugins in Presto. This addresses the gap where users configuring connectors (such as the Google Sheets connector) are not informed that the corresponding plugin must be deployed before configuring a catalog.

Resolves: #24381 and #24365

Motivation and Context

The current documentation for deploying plugins is sparse (only 6 lines in the SPI Overview page under Developer Guide), difficult to discover, and lacks cross-references from connector documentation pages. This causes users to configure connectors without deploying the required plugin, resulting in Presto failing to start with a cryptic "No factory for connector" error.

Impact

There is no performance impact

Test Plan

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Release Notes

== NO RELEASE NOTE ==

Summary by Sourcery

Update and expand documentation for deploying custom plugins and cross-reference it from relevant pages.

Documentation:

• Add a dedicated installation guide page for deploying custom plugins and connectors.
• Improve the SPI overview documentation around plugin deployment requirements.
• Cross-link connector documentation, including the Google Sheets connector, to the custom plugin deployment instructions.
• Update the main installation guide to reference the new custom plugin deployment documentation.

[sourcery-ai]

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[sourcery-ai]

Hey - I've left some high level feedback:

• Since you’ve split out deploy-custom-plugins.rst from spi-overview.rst, consider centralizing the detailed deployment steps in the new page and keeping spi-overview.rst to a short conceptual summary with a clear link, to avoid future duplication and drift between the two.
• Verify that the new installation/deploy-custom-plugins.rst page is included in the relevant Sphinx toctrees (e.g., main installation or developer docs TOCs) so it’s discoverable in the rendered docs, not just reachable via deep links from connector pages.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Since you’ve split out `deploy-custom-plugins.rst` from `spi-overview.rst`, consider centralizing the detailed deployment steps in the new page and keeping `spi-overview.rst` to a short conceptual summary with a clear link, to avoid future duplication and drift between the two.
- Verify that the new `installation/deploy-custom-plugins.rst` page is included in the relevant Sphinx toctrees (e.g., main installation or developer docs TOCs) so it’s discoverable in the rendered docs, not just reachable via deep links from connector pages.

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[steveburnett]

Looks good, thanks! Just two nits of formatting, and a reorganization of some of the information that can be done as a later PR.

[steveburnett]

LGTM! (docs)

Pull updated branch, new local doc build, looks good. Thanks!

[NivinCS]

Thanks for the change, LGTM.

[Dilli-Babu-Godari]

Thanks. LGTM!