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

[eedugon]

Looks great! I've added some suggestions for the explanations and descriptions around component templates and index templates.

btw, I've also found this insteresting blog that I'm not sure if it could make sense to link also: https://www.elastic.co/search-labs/blog/index-composable-templates

In this doc we also mention legacy templates but we don't give any context or link..... so I don't know how best to address this. I don't even know if we have the old legacy templates behavior documented (this is an old doc about legacy templates).

[eedugon]

Can we add a link in legacy templates? The concept of legacy templates doesn't appear in the doc, and might be difficult for a reader to understand what's about.

[kilfoyle]

Thanks for providing that link. In commit 24fb6bf I changed this line to:

Composable index templates take precedence over any legacy templates, which were deprecated in {{es}} 7.8.

[kilfoyle]

Thanks a lot for the feedback @eedugon!

btw, I've also found this insteresting blog that I'm not sure if it could make sense to link also: https://www.elastic.co/search-labs/blog/index-composable-templates

Definitely. I added a tip at the bottom of the intro section:

In this doc we also mention legacy templates but we don't give any context or link..... so I don't know how best to address this. I don't even know if we have the old legacy templates behavior documented (this is an old doc about legacy templates).

Thanks for the link to that page. I added the link and mentioned that legacy templates are deprecated.

Let me know if you think those changes are okay. :-)