Add doc-site-branch to version.asciidoc

[AndersonQ]

What does this PR do?

Fixes the docs for the main branch

Why is it important?

Because the links on elastic.co are broken:

doesn't work: https://www.elastic.co/guide/en/beats/metricbeat/main/metricbeat-module-consul.html
works: https://www.elastic.co/guide/en/beats/metricbeat/master/metricbeat-module-consul.html

Checklist

• [ ] My code follows the style guidelines of this project
• [ ] I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

How to test this PR locally

N/A

Related issues

N/A

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @AndersonQ? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

💚 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: 2023-06-08T15:35:51.094+0000

• Duration: 69 min 36 sec

Test stats 🧪

Test	Results
Failed	0
Passed	27333
Skipped	2009
Total	29342

💚 Flaky test report

Tests succeeded.

🤖 GitHub comments

Expand to view the GitHub comments

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

• /test : Re-trigger the build.

• /package : Generate the packages and run the E2E tests.

• /beats-tester : Run the installation tests with beats-tester.

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

[ycombinator]

Is this change enough? Does there need to be another change where doc-site-branch is used?

For example, in the Agent repo, doc-site-branch is set over here: https://github.com/elastic/elastic-agent/blob/409682e5efba99f6b4ff034a503b1d0827a67b0b/version/docs/version.asciidoc#L5

And then it is used over here: https://github.com/elastic/elastic-agent/blob/409682e5efba99f6b4ff034a503b1d0827a67b0b/dev-tools/mage/settings.go#L379