Fix link to symbols in docs

[what-the-diff[bot]]

• Updated the version of nbdev_mkdocs to 0.2.1rc0
• Added a new file doc_links_utils which contains functions for fixing sym links in markdown files generated by nbdev
• Added a new function to fix sym links in the markdown files.
• Updated all references of _fix_sym_links() with doc_links_utils.fix_sym().
• Removed unused functions from mkdocs module and moved them to doc-link utils helper class: