doc(docs/extras/tactic_writing) add cheap method (#2198)
* doc(docs/extras/tactic_writing) add cheap method
About 50% of my personal use cases for writing tactics are just because I want a simple way of stringing several tactics together, so I propose adding this so I will know where to look when I realise I can't remember the syntax.
* style fixes
* Update tactic_writing.md
* Update tactic_writing.md
* Update docs/extras/tactic_writing.md
Co-authored-by: Rob Lewis <Rob.y.lewis@gmail.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
