docs(signing): add table of contents and expand verified commits section

[ParthShikhare19]

Description:
Improve the commit signing guide in [signing.md] for better usability and clarity, especially for first-time contributors.

Add table of contents to [signing.md]
Replace "Why Commit Signing?" section with expanded "Achieving Verified Commits" section
Emphasize requirement for both -s and -S flags in commits
Add "Additional Resources" section linking to other documentation
Related issue(s):

Fixes #455

Notes for reviewer:
This update enhances the documentation to make it easier for new contributors to understand and achieve verified commits, reducing barriers to PR submission.

Checklist

Documented (Code comments, README, etc.)
Tested (unit, integration, etc.)

[exploreriii]

Excellent PR, final tidy up please and we are ready to go

[exploreriii]

One more thing. Your commits must be verified! They are not
https://github.com/hiero-ledger/hiero-sdk-python/pull/462/commits

[ParthShikhare19]

Then what can I do ?

[exploreriii]

Read this section please Achieving Verified Commits

Essentially, git commit -S -s -m "chore: your message"
for each commit
you need a GPG key set up

[ParthShikhare19]

@exploreriii I passed all cases still it shows merging is blocked

[exploreriii]

Merging is blocked, your commits must achieve verified status and they are not. See here, there is no green label to it.
https://github.com/hiero-ledger/hiero-sdk-python/pull/462/commits
You can see the status verification of your commits by doing git log --pretty="%h %G? %aN %s"
Suggest to soft revert your commit and re-commit with -S and -s as described in signing.md

[ParthShikhare19]

I added GPG key and I verify each and every commit still this happen
can you do somthing from your side to solve this problem??

[exploreriii]

did you do
git commit -s -S -m "chore: your commit message" to each commit
additionally, push your changes if they are now signed
signing.md informs you how to do it

[ParthShikhare19]

Yes I have done that already

[exploreriii]

You need to push your changes, right now your only commit was yesterday
https://github.com/hiero-ledger/hiero-sdk-python/pull/462/commits

[exploreriii]

Hi @ParthShikhare19 if you are still stuck on this, please contact me on discord and I will be happy to help

[exploreriii]

Hi @ParthShikhare19 while I can solve your conflict with the changelog, I cannot verify the commits for you and this is a requirement. Please read signing.md to achieve verified commits and please get in touch!

If you instead choose not to progress with this issue, can I take on your changes and submit it myself? I will have to change the signing though.