DOC: Finish munging howto fixup, other clean-ups

[NickleDave]

• Remove doc/data/r6paq/b17_concat_dtw.TextGrid because it's 11 MB and takes forever to load with the new TextGrid class
• Fix toctree in doc/howto.md
• Fix links in howto/convert-simple-seq.md
• Fix download links in doc/howto/munging-annotations-textgrid.md

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (21a2e51) 93.91% compared to head (ab20a36) 93.91%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #252 +/- ## ======================================= Coverage 93.91% 93.91% ======================================= Files 43 43 Lines 1659 1659 ======================================= Hits 1558 1558 Misses 101 101 ``` Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None)

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