chore(docs): Add banner and newsletter signup (SCE-29)

[kaloster]

Added back bottom banner and newsletter signup

Based on this reverted PR

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.36%. Comparing base (dc4eb1f) to head (072f789). Report is 3 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1286 +/- ## ========================================== - Coverage 92.68% 91.36% -1.33% ========================================== Files 38 82 +44 Lines 2953 6482 +3529 ========================================== + Hits 2737 5922 +3185 - Misses 216 560 +344 ``` | [Flag](https://app.codecov.io/gh/chanzuckerberg/cellxgene-census/pull/1286/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=chanzuckerberg) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/chanzuckerberg/cellxgene-census/pull/1286/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=chanzuckerberg) | `91.36% <ø> (-1.33%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=chanzuckerberg#carryforward-flags-in-the-pull-request-comment) to find out more.

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[ivirshup]

Just touched base with Nikhil, this PR still needs some work.

Interestingly this will still drop cookies so maybe we should look back into readthedocs if cookies can be okay sometimes.

[kaloster]

@ivirshup Please take another look - added doNotTrack cookie as well - as defined here and also as outlined here