Reorder executor tables to surface actively developed executors first

What does this MR do?

Reorders content in the executors index page to make it easier for users to identify and adopt the actively developed executors (Docker, Docker Autoscaler, Instance, Kubernetes).

Changes

Structure and organization

• Added ## Actively developed executors heading to group Docker, Docker Autoscaler, Instance, and Kubernetes descriptions
• Added ## Executors in maintenance mode heading to group Shell, SSH, Custom, and Docker Machine descriptions, with a brief explanation of what maintenance mode means
• Moved executor descriptions to appear before the compatibility chart
• Moved the Git requirements section to the end of the page and renamed it from "Prerequisites for non-Docker executors" to "Git requirements for non-Docker executors"

Executor lists

• Reordered the non-active executor list and placed Docker Machine last, immediately before its deprecated label, to visually separate it from the others

Flowchart

• Moved the flowchart above the selection table so users reach the faster decision tool first
• Added (maintenance mode) label to the SSH node in the flowchart
• Added a warning callout after the flowchart to flag the SSH executor's maintenance mode status and suggest the Shell executor as an alternative for local shell-based builds

Tables

• Reordered columns in the selection table and compatibility chart so actively developed executors (Docker, Docker Autoscaler, Instance, Kubernetes) appear first
• Added a maintenance mode note above both the selection table and compatibility chart
• Renumbered footnotes in the selection table to follow reading order based on the new column arrangement
• Renumbered footnotes in the supported systems by shells table to follow reading order

Related issues

#39342 (closed)

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