Link current stack version instead feature releases.

[mashhurs]

Description

When we support stack versions in VPR docs, we used (through our discussion) central release repo [master branch] to link. However, we checked that the master points to future releases, so that VPR docs should catch current stack release versions.
This PR introduces to replace master branch with current branch. Note that, current branch doc links to the current branch version doc, so that our logic pulls current version doc and retrieve stack versions from there.

Closes #74

Test

• Manually running the VPR scrip on local and check if doc-tool can fetch current stack versions:
  Command: bundle exec ruby versioned_plugins.rb --repair --skip-existing --dry-run --output-path=../
  Output log:

mik$ bundle exec ruby versioned_plugins.rb --repair --skip-existing --dry-run --output-path=../
not using a github token
Current GitHub rate limit: 46/60
Warning! Api rate limit is close to being reached, this script may fail to execute
fatal: destination path '../logstash-docs' already exists and is not an empty directory.
Already on 'versioned_plugin_docs'
Logstash version: 8.1.3
ECS version: 8.1
writing to ../logstash-docs

[kaisecheng]

LGTM

[karenzone]

LGTM