add nginx reverse proxy examples

[HeinrichAD]

To note this quite often discussed topic once.

This PR provides 2 examples:

• Nginx simple reverse proxy example
• Nginx reverse proxy example with defined location directive (different context path)

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Last seen in PR #205.
In this regard, I would like to ask @kaztechSolutions for feedback. Blow it all out of the water. Every spelling mistake, bad phrasing, incomprehensible passages. Maybe it doesn't work for you at all or you have other/more comments. This should be a cooperative work.

Also, maybe a more experienced Nginx users should also review this.

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@arnaudroques I am not 100% sure but:

• closes CSS and JS files are retrieved via http, even though plantuml is served via https (since v1-2021-10) #205
• closes Reverse proxy issue due to absolute http urls used for image #163
• closes Use PlantUML behind a reverse proxy #79
• closes Redirections with absolute urls #49
• and maybe more