Update index.md - Fixing a minor typo on the postgreSQL setup instructions

See merge request gitlab-org/gitlab-ce!30437

Many code blocks are 4spaced, and they render in GitLab
without coloring as a result, even though they are
fenced with a language label. If in a list, other items
will render as being in a code block too, even if not
meant to. This fixes all these issues, and cleans up
minor white space issues in install and integration docs

Update the installation link

See merge request gitlab-org/gitlab-ce!30489

Updating redirected links in the development, install, policy
raketasks and university dirs. Should now link to final
URL to avoid redirect hops

The installation link has changed from /installation/ to /install/. This
commit will update all the remaining links that are missing.

Previously, we used brackets to denote the tier badges,
but this made Kramdown, the docs site Markdown renderer,
show many warnings when building the site. This is now
fixed by using parentheses instead of square brackets.

This was caused by [PREMIUM] looking like a link to
Kramdown, which couldn't find a URL there.

See:
- https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/484
- https://gitlab.com/gitlab-org/gitlab-ce/issues/63800

Line 170(make install) causes permission error.
This command should execute with sudo.

Change assorted absolute links to ce and ee docs
to relative links

Add backticks around square brackets, or fix links, as necessary.