[Docs] Add Index basics page

[wajihaparvez]

Closes #369
Preview here

Summary

What I changed:

• Combined content from the three mapped pages
• Altered the heading structure to make sense of the added content and improve readability i.e.: divided the page into H2: Index components and H2: Index management + changed all other headings to H3 and moved them under the relevant H2
• Drafted a new Manage component templates subsection to be consistent with the UI/screenshots
• Cut down extra descriptive content from subsections to limit this page to Index basics only, but linked out to relevant pages to compensate. The additions to the Mapping page are from one of the raw migrated files that I think made more sense there than on Index basics
• Deleted links to Enriching your data examples from the Manage enrich policies subsection because I don't think that's relevant to this page

I'll delete the code comments after this is reviewed. @colleenmcginnis @kilfoyle let me know what you think of these decisions. I'm open to all insight/suggestions 😄

[colleenmcginnis]

This approach makes sense to me! I left a couple comments below that are more technical than editorial.

[colleenmcginnis]

The images in this PR aren't showing up for me in the preview... I'm not sure, but this might help.

Suggested change

	:::{image} /images/serverless-index-management-indices.png
	:::{image} ../../images/serverless-index-management-indices.png

[wajihaparvez]

I'm not sure what the issue was but when I was adding the ../../ on Friday, the deployment was failing. I can see the images in the preview now, could you confirm if that's true for you as well?

[colleenmcginnis]

Ah I had to hard refresh to get the page built with the latest docs-builder update (including the redesign). After that I can see the images. 👍

[colleenmcginnis]

✨