documentation: fixes to source installation instructions

What does this MR do?

Updates to source installation instructions.

Gitaly needs to be installed (and running, see #47483 (closed)) before database initialization.

Gitaly installation fails because of /home/git/repositories argument in

sudo -u git -H bundle exec rake "gitlab:gitaly:install[/home/git/gitaly,/home/git/repositories]" RAILS_ENV=production

Related issues

Closes #47483 (closed)
Gitaly cannot be started because init script is missing, there's no init script just for Gitaly

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards