[Meta][Visualizations] Improve depth of Lens documentation

[florent-leborgne]

Issue

Lens docs are outdated and options are missing.
It's also limited in the way each type of visualization is described & documented.
It's missing best practices.

What's needed

As part of this issue, the following actions will be taken:

• Template out a common docs structure and approach for the various visualization types. 1 page per visualization type is the suggested approach. [Template]
• Write a page for each type of visualization according to ^^ defined template + add any specifics (like layers for bar and area charts): Bar, Line, Area, Metric, Table, Pie, Gauge, Heat map, Waffle, Region map, Treemap, Tag cloud, Mosaic, Legacy metric
• Make a page dedicated to time series
• Document best practices for building a chart - Some can be found here https://eui.elastic.co/docs/dataviz/ (this is also public, no need to take everything, just what feels the most relevant - we can also link to this from the docs)
• Add content to help users choose an effective chart type for their use cases

To take into account

• Check impacts on IA/page structure and existing content. Some content might move.
• Check if some parts can be reused using snippets.
• Check SEO implications as we've recently got indexing issues with templated content.
• Make sure that all settings are covered either in relevant chart type pages or in separate pages for common Lens principles. Some major settings missing docs today are: Color mapping, Breakdown, Rank by.
• Relevant examples must be added and explained. Using the Obs robot to get good example data would be a good approach to show realistic examples.

Existing related issues

• https://github.com/elastic/platform-docs-team/issues/467
• https://github.com/elastic/platform-docs-team/issues/469

cc @markov00

[florent-leborgne]

Re-qualifying this as a meta issue so it's structured and planned accordingly