Various 5.0 fixes

[confluence]

Here are my suggested fixes for the release (all in the installation section):

• Use the base RTD URL or latest (needed for internal pages) when referring to the stable release (we can decide what to do for the beta before the next beta). As long as we update the default and latest correctly on RTD, this means that we can stop changing the versions here and running into copy & paste errors.
• Formatting: consistent use of paragraph elements
• Removed details about old RPM repo and some subheadings that aren't used elsewhere
• Added / tweaked comments about similar platforms in the different package sections
• Added link to manual to RPM section
• Wording tweaks for grammar and to fit tabbed sections within the maximum allowed height
• Removed obsolete tab about CentOS 7 and renumbered other tabs