Create new CLI command "nbdev mkdocs readme"

[harishmohanraj]

Closes #178

[what-the-diff]

PR Summary

• Updated the .secrets.baseline file
  Some values in this file were updated, notably the hash and the time of creation. This is a routine task for security purpose.

• Modified the README.md file
  The title and a link in the User Guide section were updated. The "Install" section was removed and replaced by a new section "Prepare changes" which includes a guide on how to prepare code changes.

• Changed the software version
  The version of the software was updated from "0.6.0" to "0.7.0rc0", indicating enhancements have been made.

• Updated commands in the _cli.py file
  Some command descriptions were updated for better clarity for users. A new command called readme which updates the README.md file was added. Also, a new function to generate a custom social sharing image was added.

• Added new files for generating documentation
  Two new files, nbdev_mkdocs/_components/__init__.py and nbdev_mkdocs/_components/cli_docs_generator.py, were added which contain fresh code for generating documentation.

• Improved link functions
  In the nbdev_mkdocs/_helpers/doc_links_utils.py file, an existing function was improved by adding a new parameter. Moreover, a new function was added to fix default system links.

• Enhanced symbol links
  In the nbdev_mkdocs/_modidx.py file, additional symbol links were added to the syms dictionary, to enhance the usefulness of symbol reference.

• Added multiple enhancements to CLI_Docs_Generator.ipynb file
  Multiple lines of code have been added to the CLI_Docs_Generator.ipynb file to enhance its functionality and usability.

• Updated nbs/index.ipynb file
  The nbs/index.ipynb file was updated multiple times to improve its effectiveness.