Add documentation for HA cluster support features

[barthisrael]

This PR adds documentation for all the features that have been as part of the work of making it easier to integrate Barman with HA clusters.

We perform the following changes to the web docs / man:

• Add cluster configuration option to man 5
• Add model configuration option to man 5
• Include the Model scope to all options that configuration models support
• Add information about configuration models under the Integration with cluster management systems section of the web docs
• Add information about configuration models and examples under the Configuration section of the web docs
• Add documentation about the barman config-switch new CLI command in the web docs

Some changes to the documentation related with wal_conninfo and wal_streaming_conninfo have already been applied through #868, and are only pending to be merged.

Besides that we applied some standardization on all configuration options in man (doc/barman.5.d/50-* files):

• Have an empty new line at the end -- some of them were missing this;
• Have at most 80 characters per line -- some of them were not following this rule;
• Have a Scope: line near the end, after the description of the configuration option: it specifies which is the scope of the config option, i.e., if it can be applied to one or more scopes among Global, Server and Model. This information used to be present in the man 5 files already, however they were part of the descritpion. Now we have some emphasis with a separate paragraph which follows the same standard over all files.

Also, there are more docs to come, once we finish other work related with HA cluster features which is still pending.

References: BAR-136.

[edb-sonar-app[bot]]

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[barthisrael]

We will merge this once #868 and #873 get merged because this PR includes documentation for those PRs. We decided to merge this PR and follow-up with documenting future work related with models through the specific PRs that are coming.

[barthisrael]

I'm bypassing protections and merging this anyway (Martin is currently on PTO and his thoughts have been applied to the PR).