[DOC] Document the clang-format flow

[Will add to CONTRIBUTING.md] How to install clang-format 18 on a Debian box

sudo apt-get update -y
sudo apt-get upgrade -y
sudo apt-get install software-properties-common -y
wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | gpg --dearmor | sudo tee /usr/share/keyrings/llvm-toolchain.gpg > /dev/null
echo "deb [signed-by=/usr/share/keyrings/llvm-toolchain.gpg] http://apt.llvm.org/$(lsb_release -cs)/ llvm-toolchain-$(lsb_release -cs)-18 main" | sudo tee /etc/apt/sources.list.d/llvm.list
sudo apt-get update -y
sudo apt-get install clang-format-18 -y

How to run clang-format-check action locally

1. get act
2. commit the changes locally git commit -a -s
3. under valkey repo root directory, run pingxie@penguin ~/valkey (clang-format)>sudo act -j clang-format-check
4. here is a failure output

[Clang Format Check/clang-format-check]   ✅  Success - Main Set up Clang
[Clang Format Check/clang-format-check] ⭐ Run Main Run clang-format
[Clang Format Check/clang-format-check]   🐳  docker exec cmd=[bash --noprofile --norc -e -o pipefail /var/run/act/workflow/clang-format.sh] user= workdir=
[Clang Format Check/clang-format-check]   ✅  Success - Main Run clang-format
[Clang Format Check/clang-format-check]   ⚙  ::set-output:: diff=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
[Clang Format Check/clang-format-check] ⭐ Run Main Check for formatting changes
[Clang Format Check/clang-format-check]   🐳  docker exec cmd=[bash --noprofile --norc -e -o pipefail /var/run/act/workflow/3.sh] user= workdir=
| Code is not formatted correctly. Here is the diff:
| diff --git a/src/util.c b/src/util.c
| index 8bd9a286c..2ea9fe384 100644
| --- a/src/util.c
| +++ b/src/util.c
| @@ -110,7 +110,11 @@ static int stringmatchlen_impl(const char *pattern,
|                      if (pattern[0] == string[0]) match = 1;
|                  } else if (pattern[0] == ']') {
|                      break;
| -                } else if (patternLen == 0) { pattern--; patternLen++; break; } else if (patternLen >= 3 && pattern[1] == '-') {
| +                } else if (patternLen == 0) {
| +                    pattern--;
| +                    patternLen++;
| +                    break;
| +                } else if (patternLen >= 3 && pattern[1] == '-') {
|                      int start = pattern[0];
|                      int end = pattern[2];
|                      int c = string[0];
[Clang Format Check/clang-format-check]   ❌  Failure - Main Check for formatting changes
[Clang Format Check/clang-format-check] exitcode '1': failure
[Clang Format Check/clang-format-check] 🏁  Job failed

5. here is a successful run

[Clang Format Check/clang-format-check]   ✅  Success - Main Set up Clang
[Clang Format Check/clang-format-check] ⭐ Run Main Run clang-format
[Clang Format Check/clang-format-check]   🐳  docker exec cmd=[bash --noprofile --norc -e -o pipefail /var/run/act/workflow/clang-format.sh] user= workdir=
[Clang Format Check/clang-format-check]   ✅  Success - Main Run clang-format
[Clang Format Check/clang-format-check] Cleaning up container for job clang-format-check
[Clang Format Check/clang-format-check] 🏁  Job succeeded

Originally posted by @PingXie in https://github.com/valkey-io/valkey/issues/538#issuecomment-2134315396