[Lens] Page for Line charts

[alaudazzi]

Fixes #2540.

Here is a draft page for line charts.

• Optimize structure for readability
• Analyze competition, check discuss forums, test product and check existing materials to decide on optimal content depth
• Extract best practices from EUI's dataviz guidelines
• Test current content - doesn't mean the doc is exhaustive, just that what's in there is correctly reflecting the product; there may be more nuance to add edge cases.
• Have doc reviewed for completeness and accuracy, in particular the examples

This page is currently hidden and will be enabled once we have a batch of pages for metrics, areas, bars, time series, and other visualizations.
We'll remove duplicated content from the main Lens page when those pages go live.

[github-actions]

Vale Linting Results

Summary: 2 warnings, 9 suggestions found

⚠️ Warnings (2)

File	Line	Rule	Message
explore-analyze/visualize/charts/line-charts.md	79	Elastic.Latinisms	Latin terms and abbreviations are a common source of confusion. Use 'versus' instead of 'vs'.
explore-analyze/visualize/charts/line-charts.md	95	Elastic.Latinisms	Latin terms and abbreviations are a common source of confusion. Use 'versus' instead of 'vs'.

💡 Suggestions (9)

File	Line	Rule	Message
explore-analyze/visualize/charts/line-charts.md	56	Elastic.Acronyms	'EUI' has no definition.
explore-analyze/visualize/charts/line-charts.md	56	Elastic.WordChoice	Consider using 'refer to (if it's a document), view (if it's a UI element)' instead of 'See', unless the term is in the UI.
explore-analyze/visualize/charts/line-charts.md	62	Elastic.WordChoice	Consider using 'refer to (if it's a document), view (if it's a UI element)' instead of 'See', unless the term is in the UI.
explore-analyze/visualize/charts/line-charts.md	75	Elastic.WordChoice	Consider using 'refer to (if it's a document), view (if it's a UI element)' instead of 'see', unless the term is in the UI.
explore-analyze/visualize/charts/line-charts.md	183	Elastic.Acronyms	'SLO' has no definition.
explore-analyze/visualize/charts/line-charts.md	191	Elastic.Acronyms	'SLO' has no definition.
explore-analyze/visualize/charts/line-charts.md	231	Elastic.WordChoice	Consider using 'refer to (if it's a document), view (if it's a UI element)' instead of 'see', unless the term is in the UI.
explore-analyze/visualize/charts/line-charts.md	231	Elastic.Acronyms	'EUI' has no definition.
explore-analyze/visualize/supported-chart-types.md	32	Elastic.WordChoice	Consider using 'refer to (if it's a document), view (if it's a UI element)' instead of 'see', unless the term is in the UI.