Renamed rail to sidebar in Layout.

[AlexSkrypnyk]

We introduced "rail" within Layout component in 1.8.

During several discussions with users, the "rails" was not understood as clearly as a "sidebar".

A quick search on Drupal.org (as an example) has shown 23k results for "sidebar" and 800 results for "rail". This is an indication that "sidebar" is a commonly used term.

Below is an additional analysis:

In web design, the terms "sidebar" and "rail" are used to describe vertical sections that typically flank the main content area, but they have different connotations and uses.

The terms "sidebar" and "rail" can have different connotations and uses, especially in web design and layout contexts.

Sidebar

• Definition: A sidebar is a vertical column that is positioned to the left or right of the main content area on a webpage. It often contains additional information, navigation links, advertisements, or other supplementary content.
• Common Usage: Sidebars are used in blogs, news sites, and various web applications to provide secondary content that complements the main content area. They are a staple in many traditional web designs.
• Position: Typically found on either the left or right side of the main content.
• Examples: Blog sidebars with categories, recent posts, advertisements, or social media links.

Rail

• Definition: A rail is a term used similarly to a sidebar but can have a slightly different context, often implying a slimmer, more streamlined area adjacent to the main content. The term is sometimes used to convey a more modern or minimalist design.
• Common Usage: Rails are often used in web applications or modern websites where design simplicity and streamlined content presentation are prioritized. They might contain secondary navigation, promotional content, or calls to action.
• Position: Also positioned alongside the main content, either on the left or right, but can sometimes suggest a narrower or less obtrusive design compared to a sidebar.
• Examples: Promotional rails with special offers, slim navigation menus, or social media integration in modern web designs.

Differences

• Connotation: "Sidebar" often implies a more traditional, content-rich area, while "rail" suggests a slimmer, more modern design.
• Usage Context: Sidebars are common in traditional websites and blogs, whereas rails are more often used in contemporary, minimalist web designs.
• Design: Rails may occupy less space and have a more streamlined look compared to sidebars, which can be wider and more content-heavy.

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This PR renames rail to sidebar.

We also want to reserve rail for actual Navigation rail that we can introduce in the future.

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