[D&M] Drafts maintenance content

[szabosteve]

Overview

Related to https://github.com/elastic/docs-projects/issues/353
This PR:

• adds an introductory paragraph to the Maintenance overview that lists the main topics covered in the section
• adds a "super-brief" intro to the Start and stop services section
• drafts the Start and stop ES section by merging the contents of two source files (starting-elasticsearch.md and stopping-elasticsearch.md) and adding an intro to the page
• drafts ECE maintenance intro and adds Enabling Kibana content to the page (as a temporary solution)
• drafts Maintenance activities section
• drafts Start and stop services section
• refines existing pieces.

[shainaraskas]

this looks very very good!

most of my comments are nitpicks and little typo things I saw. overall:

• Please add applies_to to all pages (the new syntax is out now)
• From overview pages, link down to the pages you're referring to
• Duplicate content about adding nodes can be removed from the "start/stop es" topic
• Noted where some small glue content and clarification about what procedures apply to what deployment types would be helpful (and hopefully quick)

[shainaraskas]

I've provided some suggestions re: changes to the "applies_to" tags. most of my reasoning is based on extrapolating other info in the page (e.g. the install methods on start/stop es are just self-managed install methods). let me know if you disagree with any of the labeling.

sry for the second round of comments - it's looking vv great, just want to make sure we have the page scopes right 🎯