Plugin doc sync doesn't linkify bare URLs in generated content

## Describe the issue

Plugin pages generated by the sync pipeline (`helper-scripts/influxdb3-plugins/port_to_docs.js`) render bare URLs as plain text instead of clickable links when the upstream plugin README includes them as plain text rather than markdown links.

Example, from the National Weather Service plugin's "API Reference" section:

```
API Reference
National Weather Service API

Base URL: https://api.weather.gov
Documentation: https://www.weather.gov/documentation/services-web-api
```

None of these URLs render as links.

##### Relevant URLs
- https://test2.docs.influxdata.com/pr-preview/pr-7754/influxdb3/core/plugins/library/official/nws-weather/
- `content/shared/influxdb3-plugins/plugins-library/official/nws-weather.md` (generated region, lines ~396-404)
- `helper-scripts/influxdb3-plugins/port_to_docs.js`

## Root cause

`port_to_docs.js` transforms the upstream README with targeted regex/string
replacements (for example `convertRelativeLinks()` for cross-plugin links).
It has no general autolink step, so bare URLs in upstream prose pass through
unlinked into the generated region between
`` markers.

Per `content/shared/influxdb3-plugins/plugins-library/official/AGENTS.md`,
this content is regenerated on every sync run, so hand-fixing the rendered
`.md` files is not a durable fix — it needs to happen in the transform.

## Suggested fix

Add an autolink pass in `port_to_docs.js` (or reuse a markdown library's
autolinker) that wraps bare `http(s)://` URLs in the generated region as
markdown links before writing output, similar to the existing
`convertRelativeLinks()` step for relative plugin links.

Start in helper-scripts/influxdb3-plugins/port_to_docs.js, especially the existing convertRelativeLinks() step and the generated-content boundaries. Run the plugin sync pipeline and inspect content/shared/influxdb3-plugins/plugins-library/official/nws-weather.md. Done means bare http(s):// URLs from upstream prose render as clickable markdown links without hand-editing generated files.

Written by the indexing model from the issue text.