Add some developer documentation about profiling

docs/dev/Profiling.md

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+# Profiling Lazygit
+
+If you want to investigate what's contributing to CPU or memory usage, start
+lazygit with the `-profile` command line flag. This tells it to start an
+integrated web server that listens for profiling requests.
+
+## Save profile data
+
+### CPU
+
+While lazygit is running with the `-profile` flag, perform a CPU profile and
+save it to a file by running this command in another terminal window:
+
+```sh
+curl -o cpu.out http://127.0.0.1:6060/debug/pprof/profile
+```
+
+By default, it profiles for 30 seconds. To change the duration, use
+
+```sh
+curl -o cpu.out 'http://127.0.0.1:6060/debug/pprof/profile?seconds=60'
+```
+
+### Memory
+
+To save a heap profile (containing information about all memory allocated so
+far since startup), use
+
+```sh
+curl -o mem.out http://127.0.0.1:6060/debug/pprof/heap
+```
+
+Sometimes it can be useful to get a delta log, i.e. to see how memory usage
+developed from one point in time to another. For that, use
+
+```sh
+curl -o mem.out 'http://127.0.0.1:6060/debug/pprof/heap?seconds=20'
+```
+
+This will log the memory usage difference between now and 20 seconds later, so
+it gives you 20 seconds to perform the action in lazygit that you are interested
+in measuring.
+
+## View profile data
+
+To display the profile data, you can either use speedscope.app, or the pprof
+tool that comes with go. I prefer the former because it has a nicer UI and is a
+little more powerful; however, I have seen cases where it wasn't able to load a
+profile for some reason, in which case it's good to have the pprof tool as a
+fallback.
+
+### Speedscope.app
+
+Go to https://www.speedscope.app/ in your browser, and drag the saved profile
+onto the browser window. Refer to [the
+documentation](https://github.com/jlfwong/speedscope?tab=readme-ov-file#usage)
+for how to navigate the data.
+
+### Pprof tool
+
+To view a profile that you saved as `cpu.out`, use
+
+```sh
+go tool pprof -http=:8080 cpu.out
+```
+
+By default this shows the graph view, which I don't find very useful myself.
+Choose "Flame Graph" from the View menu to show a much more useful
+representation of the data.