Add Replace Backreferences section to Regex Features page

[davidwendt]

Adds Replace Backreferences section to the Regex Features page to help document the replacement template patterns for cudf::strings::replace_with_backref API.

Also, adds cudf::strings::split_re and cudf::strings::split_record_re to the regex APIs listed at the top of the page.

And add a link to regex_flags where the line anchors and the dot pattern character are described.

[codecov]

Codecov Report

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@@               Coverage Diff               @@
##             branch-22.06   #10560   +/-   ##
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  Hits                    ?    19253           
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[davidwendt]

@gpucibot merge