Make bash autocomplete instructions easier to find

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/556)
In GitLab by [[Gitlab user @Qinusty]](https://gitlab.com/Qinusty) on Aug 6, 2018, 11:17

## Background

[//]: # (Provide a background or the root/source that justifies this task or action.)

[Buildstream autocompletion instructions](http://buildstream.gitlab.io/buildstream/install_linux_distro.html#bash-completions) are quite deep within the install documentation.

## Task description

[//]: # (Short summary of the action to be executed)

* [ ] Make instructions easier to find either by moving or linking to from elsewhere.

## Acceptance Criteria

[//]: # (Acceptance criteria should follow the S.M.A.R.T. principle https://en.wikipedia.org/wiki/SMART_criteria )

Autocomplete instructions should be easy to find for newcomers to the project.

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Start with the Bash completions section in install_linux_distro.html, linked from the issue, and inspect how newcomers currently reach it. Make the instructions easier to discover by moving them or adding a clear link elsewhere in the documentation, then verify that a newcomer can locate the Bash autocomplete instructions without searching deeply through the install page.

Written by the indexing model from the issue text.