pgsql: Add guidance on making documentation SVG images responsive

Add guidance on making documentation SVG images responsive

Recently added guidance on adding SVG images to the documentation
sources lacks advice on making the images responsive when rendered
in a variety of media types and viewports. Add some.

Patch by Jonathan Katz with some editorialization by me.

Author: Jonathan Katz
Discussion: https://postgr.es/m/6358ae6f-7191-a02b-e7b5-68050636ae71@postgresql.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/7dc78d8ef3e62f7e06d7767c63dcede048377b9a

Modified Files
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doc/src/sgml/images/README | 7 +++++++
1 file changed, 7 insertions(+)