Add composite template docs

[everettraven]

Description

• Adds documentation on the composite template introduced to opm via the opm alpha render-template composite subcommand

[grokspawn]

Looks like the linter no longer supports flags we use in the linter workflow (:latest was updated 9 days ago):

Unable to find image 'mtlynch/htmlproofer:latest' locally
latest: Pulling from mtlynch/htmlproofer
63b65145d645: Already exists
ea9e29845fca: Pull complete
b9b65e4e1280: Pull complete
2da0d7c94605: Pull complete
490cc9f9b1d8: Pull complete
a0a491f3d1e9: Pull complete
8fefe0f687c2: Pull complete
Digest: sha256:f72326c5a6cfe52b9268faf5d3f686b387409f2312b98cfb39b6644fbaeca7e1
Status: Downloaded newer image for mtlynch/htmlproofer:latest
/usr/local/bundle/gems/html-proofer-5.0.6/lib/html_proofer/configuration.rb:68:in `parse_cli_options': invalid option: --empty-alt-ignore (OptionParser::InvalidOption)
    from /usr/local/bundle/gems/html-proofer-5.0.6/lib/html_proofer/cli.rb:14:in `run'
    from /usr/local/bundle/gems/html-proofer-5.0.6/exe/htmlproofer:14:in `block in <top (required)>'
    from /usr/local/lib/ruby/3.2.0/benchmark.rb:311:in `realtime'
    from /usr/local/bundle/gems/html-proofer-5.0.6/exe/htmlproofer:14:in `<top (required)>'
    from /usr/local/bundle/bin/htmlproofer:25:in `load'
    from /usr/local/bundle/bin/htmlproofer:25:in `<main>'

[oceanc80]

/lgtm