[8.0] Added non-agent-specific docs on span compression (backport #6438)

[mergify]

This is an automatic backport of pull request #6438 done by Mergify.

---

Mergify commands and options

More conditions and actions can be found in the documentation.

You can also trigger Mergify actions by commenting on this pull request:

• @Mergifyio refresh will re-evaluate the rules
• @Mergifyio rebase will rebase this PR on its base branch
• @Mergifyio update will merge the base branch into this PR
• @Mergifyio backport <destination> will backport this PR on <destination> branch

Additionally, on Mergify dashboard you can:

• look at your merge queues
• generate the Mergify configuration with the config editor.

Finally, you can contact us on https://mergify.com

[apmmachine]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Start Time: 2021-11-25T00:56:29.186+0000

• Duration: 42 min 35 sec

• Commit: 6c6f2b7

Test stats 🧪

Test	Results
Failed	0
Passed	5586
Skipped	19
Total	5605

🤖 GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• /hey-apm : Run the hey-apm benchmark.

• /package : Generate and publish the docker images.

• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)