docs: Add "deprecating soon" notices

[bmorelli25]

Summary

The main purpose of this PR was:

• Adds deprecation notices to all "Legacy" documentation pages and links to relevant APM integration/Fleet docs where applicable (except those pages that will be moved in docs: Add upgrading documentation #6709, like upgrading, breaking changes, and agent/server compact)

Along the way (I had to look at every single file in the legacy docs) I noticed a few things:

• Fixed a few small layout bugs
• Moves legacy overview content from their own pages to the top page of each legacy bucket (1) (2)
• Updates the arch diagram and component documentation (link) per docs: APM integration documentation #6287 (comment)
• Removed original APM integration docs from APM Server Reference (these)

Thought process

As you're all well aware, there's now a lot of duplicated content in the APM documentation. This duplicated content exists in three forms:

1. Content where there's a major difference between the APM integration docs and APM Server legacy docs. Example: Quick start guides.
2. Content where there are minor differences between the APM integration docs and APM Server legacy docs. Example: API docs.
3. Content where there is no difference between the APM integration docs and APM Server legacy docs. Example: Transaction sampling docs.

Why duplicate everything? The idea was to have two distinct guides—one for APM integration users, and one for legacy APM Server users. The only way this is possible is by ensuring each section has all of its own docs, even if that means duplicating some content verbatim. But this leads to a new problem—what happens when users google their way into the wrong docs? Before, it wasn't immediately clear which docs you were looking at. So, in this PR, I've added a note to the top of every page in the legacy APM Overview and the legacy APM Server Reference indicating that the method of running/installing/configuring APM Server will be deprecated in a future release. In addition, if there's related documentation for the APM integration, I've linked to it.

(soon to be) Deprecated words

THE 🥩 🥩 🥩 🥩 🥩 🥩 OF THIS PR IS HERE

I originally wanted to get rid of the word legacy, but I actually think it works well in this context. Open to any feedback and all ideas—I'm curious to read your thoughts!

apm-server/docs/notices.asciidoc

Lines 1 to 19 in 8467edf

	// For installation, get started, and setup docs
	:deprecation-notice-installation: This method of installing APM Server will be deprecated and removed in a future release. Please consider getting started with the <<apm-quick-start,Elastic APM integration>> instead.
	// Generic "running" message
	// Usually followed by a link to the corresponding APM integration docs
	// Link to upgrade docs will be added in #6709
	:deprecation-notice-data: This documentation refers to the standalone (legacy) method of running APM Server. This method of running APM Server will be deprecated and removed in a future release. Please consider upgrading to the Elastic APM integration.
	// For monitoring docs
	// Link to upgrade docs will be added in #6709
	:deprecation-notice-monitor: This documentation refers to monitoring the standalone (legacy) APM Server. This method of running APM Server will be deprecated and removed in a future release. Please consider upgrading to {fleet} and the APM integration.
	// For configuration docs
	// Link to upgrade docs will be added in #6709
	:deprecation-notice-config: This documentation refers to configuring the standalone (legacy) APM Server. This method of running APM Server will be deprecated and removed in a future release. Please consider upgrading to {fleet} and the APM integration.
	// For API docs
	// Link to upgrade docs will be added in #6709
	:deprecation-notice-api: This documentation refers to the API of the standalone (legacy) APM Server. This method of running APM Server will be deprecated and removed in a future release. Please consider upgrading to {fleet} and the APM integration.

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• Start Time: 2021-11-29T17:14:49.515+0000

• Duration: 43 min 38 sec

• Commit: d884337

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Test	Results
Failed	0
Passed	5586
Skipped	19
Total	5605

🤖 GitHub comments

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[axw]

Looks great!

[simitt]

++ This looks great! Thanks for the effort you put in here.