Change indentation format for RST (and possibly Markdown?) files

[lengau]

Per @medubelko in canonical/charmcraft#2010 (comment) - indent_size for RST files should be 2.

Remaining questions:

1. What indent_size should we have for markdown?
2. Should we be setting max_line_length to 88 for these too?

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The internal ticket has been created: https://warthogs.atlassian.net/browse/CRAFT-3853.

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[syncronize-issues-to-jira]

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The internal ticket has been created: https://warthogs.atlassian.net/browse/CRAFT-3852.

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[medubelko]

Executing this could be a good early task for JJ.