Move SRAM docs from this repo to gf180mcu_fd_ip_sram repo

[mohanad0mohamed]

• SRAM docs moved to gf180mcu_fd_ip_sram repo
• Docs are added in pull request https://github.com/google/globalfoundries-pdk-ip-gf180mcu_fd_ip_sram/pull/5

Fixes #11

[mithro]

I have reworked this pull request in a branch at https://github.com/mithro/gf180mcu-pdk/tree/move_sram

[mithro]

The key differences between my version and yours;

• Only includes changes to SRAM related files.
• Updates the gf180mcu_fd_ip_sram repository submodule at the same time as removing the content.

[mithro]

I'll merge this pull request if the documentation build looks good.

[mohanad0mohamed]

@mithro I think we still need to edit the index.rst file to include the new SRAM directory. Also I found that ip.rst is removed which was calling SRAM docs.

[mohanad0mohamed]

Also in this PR, it should close #15. but already another PR is made for it here #32.

[mohanad0mohamed]

Updated index.rst and added ip.rst

[mohanad0mohamed]

I have changed the symbolic link name from SRAM to gf180mcu_fd_ip_sram to match naming

[mithro]

I have updated this pull request. If readthedocs goes green, then I'll merge the pull request.