Fix up the 'Templates' section

[kilfoyle]

This attempts to improve the Templates docs by:

• Adding more explanation into the page rather than just linking to the API docs.
• Removing the "manage index templates" content that is duplicated; this section in the more prominent "Index basics" will be the single source of truth.
• Improving the narrative by making a clearer distinction between "Index templates" and "component templates", with separate sections for each.
• Changing the giant warning about index pattern collisions (which takes about 1/3 of the page) into a subsection of "Index templates"; this should flow better and be less jarring for readers.
• Including the API instructions for creating each kind of template, and also link to the Kibana version of those steps that are included in the Index basics page.
• Adding a tip with links to the subpages about simulating an index template and about ignore missing component templates, just to help people be aware of these cool features.

Please see preview page: Templates

@eedugon This was prompted by your suggestions here so I hope you don't mind my adding you as a reviewer. 🙏

Closes: #2056

[github-actions]

🔍 Preview links for changed docs

• manage-data/data-store/index-basics.md
• manage-data/data-store/templates.md
• manage-data/data-store/templates/index-template-management.md