Cap markdown to fix documentation builds

[kiersten-stokes]

What changes were proposed in this pull request?

This (temporarily) caps markdown in our docs requirements.txt, which was causing a test failure: see https://github.com/elyra-ai/elyra/runs/7362961491?check_suite_focus=true. The failure also occurred locally with Markdown versions 3.4 and 3.4.1

How was this pull request tested?

Tested make docs locally to ensure no error during build.

Developer's Certificate of Origin 1.1

   By making a contribution to this project, I certify that:

   (a) The contribution was created in whole or in part by me and I
       have the right to submit it under the Apache License 2.0; or

   (b) The contribution is based upon previous work that, to the best
       of my knowledge, is covered under an appropriate open source
       license and I have the right under that license to submit that
       work with modifications, whether created in whole or in part
       by me, under the same open source license (unless I am
       permitted to submit under a different license), as indicated
       in the file; or

   (c) The contribution was provided directly to me by some other
       person who certified (a), (b) or (c) and I have not modified
       it.

   (d) I understand and agree that this project and the contribution
       are public and that a record of the contribution (including all
       personal information I submit with it, including my sign-off) is
       maintained indefinitely and may be redistributed consistent with
       this project or the open source license(s) involved.

[elyra-bot]

Thanks for making a pull request to Elyra!

To try out this branch on binder, follow this link:

[ptitzler]

LGTM! Documentation build completes now successfully.