docs: Add plotly.js v4 migration guide

[camdecoster]

Description

Creates a new Guides section, moves the v3 migration guide, and ddds the plotly.js v4 migration guide.

Closes plotly/plotly.js#7831.
Closes plotly/plotly.js#7841.

Changes

• Creates Guides section
• Moves/renames v3 migration guide
• Adds v4 migration guide
• Updates existing docs to reflect v4 changes
• Adds new docs for new features

Testing

• Be on this branch
• Build the docs
• Click around the Guides and make sure everything looks okay

Notes

• The v3 guide was in the Fundamentals section, which doesn't really fit
• I added a redirect for the old v3 guide link
• The styles aren't great, but fixing that will be part of a different project

[ndrezn]

I would suggest adding some grouping by topic, like for example. Right now it's a just a long, ungrouped list so topics come and go back and forth.

Maybe something like:

1. Environment & tooling

Setup-level changes that hit before you touch a figure.

• Minimum Node.js version
• TypeScript types

2. Removed features

Things that are simply gone and need code deleted/renamed.

• Chart Studio APIs removed
• Mapbox traces and subplots removed (maybe could go in maps section? but it's top item).

3. Dependency changes

Third-party library swaps whose output shifts even with identical input — the
"your figure looks slightly different and you didn't change anything" bucket.

• Color library swap (tinycolor2 → color)
• Country name lookup (country-regex → country-iso-search)
• Sankey layout algorithm update

4. Maps & geo

By far the biggest cluster — all the MapLibre/geo framing behavior.

• Scattermap icon defaults
• Map subplots auto-frame to data
• Geo subplots auto-frame by default
• Geo fitbounds framing for antimeridian features
• Geo subplot zoom limits

5. Cartesian & shapes

The remaining rendering-behavior changes that aren't map-related.

• Shape legend marker outlines honor line.dash
• Overlaying axis tickmode defaults to 'sync'

[ndrezn]

Should be a sub header

[ndrezn]

Should be one level deeper

[emilykl]

Suggested change

	### Antimeridian data
	For data that straddles the antimeridian (points on both sides of ±180°),
	the auto-frame picks the compact crossing range rather than the
	long-way-around view. For example, `lon: [131.8855, -179]` frames as
	`[131.8855, 181]` (a ~49° span across the antimeridian) instead of the
	naive `[-179, 131.8855]` (~311° the wrong way).
	### Data that crosses the antimeridian
	For data that straddles the antimeridian (points on both sides of ±180°, in the middle of the Pacific Ocean),
	the auto-frame picks the compact crossing range rather than the
	long-way-around view. For example, `lon: [131.8855, -179]` frames as
	`[131.8855, 181]` (a ~49° span across the antimeridian) instead of the
	naive `[-179, 131.8855]` (~311° the wrong way).

Would be nice to have a visual example here